The GBA will feature some new innovations (some of which I don't even know yet), but this chart (originally by Nintendo of Japan and translated into english by me) shows just what to expect in terms of new tidbits.

1.  Shoulder L and R buttons- akin to the ones on the SNES
2.  Communications Port- used to link between GBA's, or between a GBA and a Nintendo Cube
3.  Peripheral attachment slots
4.  Green Power LED
5.  TFT screen, 512 to 32K onscreen colors, resolution of 244x160
6.  Mono Speaker
7.  Carrying strap connector
8.  Power Switch
9.  Headphone Jack
10. Volume
11. GBA cartridge housing.  Notice how much shorter (and wider) it is than the GB/GBC cartridges.
 

If you've been in a cave for the last decade, the Gameboy is the worlds first, longest lasting, and most supported portable of all time, surviving the Atari Lynx, Sega Game gear, Turboexpress, Nomad, Game.com, and Neo Geo Pocket Color.

Released in 1988 (japan) and 1989 (USA), the system has sold over 60 million units worldwide in both Gameboy and Gameboy Pocket forms. The library of games has grown to over 1000 over the last ten years despite being  the least technically impressive. 

It ran on 4 AA batteries for anywhere from 15-30 hours, and was based on the Z80 processor architecture.  The screen used 4 shade greyscale LCD.  The Pocket model, released in 1996, ran on 2 AAA batteries for 10 hours, with a higher contrast screen. 

Over the years there have been dozens of variations of the gameboy.  Counting all possible combinations of colors and sizes, I think the grand total of different North American gameboys is somewhere around 17-20.  Worldwide, over 50 variations of the GB, GBPocket and GB Color have been released.

Over 100 million units of GB hardware have been shipped since the systems inceptions

Basically, it is a gameboy on steroids with a color screen provided by sharp, an 8 MHz Z80-esque Processor (also made by Sharp) with additional main memory as welll as some other nifty features, most listed in this document. 

The GBC was released Oct. 23, 1998 in Japan, Nov. 19 in the states, Nov. 23 In Europe and most other places.  I saw first signs of the GBC in Calgary on Nov. 21.

The GBC could be compared to the atari 7800 because of its advancements over its predecessors as well as backwards compatability.

While the Gameboy Color architecture remains part of the GBA (for backwards compatibility as well as sound functionality), an entirely new chipset has been designed for this system, making it the most impressive handheld ever seen.

The system's product code is AGB-001 standing for "Advanced Game Boy", first product in the product line.  Gameboy Color's code was CGB- "Color Game Boy", Gameboy Pocket's designation was MGB meaning "Miniature Game Boy", and the original GB was DMG, which meant "Dot Matrix Game" (and coincidentally where the DMG-ICE website gets its name).
 

Yes.  The Gameboy Advance features a full GBC chipset.  So when you turn on a GBA with a GBC cartridge in the slot, it will simply boot up using the standard GBC boot (or GB color selection) screen.  All software from the old GB era also works via this method.  The cartridges will stick out of the cartridge slot due to the fact that GBA cartridges are shorter and wider than GB/GBC cartridges.
 

Processors:
32 bit Arm7 custum with embedded memory running at 16.7 MHz.  Embedded into the processor is a fully functional GBC chipset, the details of which can be found on the GBC FAQ.  GBA can be booted over a network from the "Master" GBA using a link cable in software that supports it.

RAM: 32KB work , 256K extended work,  96KB video, 16KB sound

Video: 512 color sprite/background mode, 32768 color mode for bitmap mode.  Contrary to popular belief, there is no embedded hardware for video playback.  Supports the following effects:

XY scrolling, rotational scrolling
Hardware Sprite scaling/distortion/rotation
Max 4096 sprites at one time, up to 256 on a line (sure beats the old 10 limit!)
Supports alpha blending
Hardware Transparencies
Multiple levels of Parallax

Sound:  MusyX sound system for up to 32 voices at once + 4 channel PSG sound from the Sharp Z80.  Output provided by dual 8-bit RAW PCM channels at 44.1KHz. Single Mono speaker w/headphone jack.

Controls:  8 directional D-Pad, A,B,L,R, select, and Start buttons. 

Power:  20 hours on average from 2 AA batteries, or power from AC adapter.  15 hours from official battery pack

Size:  Similar to the gameboy Color with a horizontal orientation.

Resolution: 244x160x512(32768) in GBA mode, 160x144x56 colors in GBC mode

Sprites: 4096, 256 per line (max)

Tiles: Unknown

Cart Size:  Max 512Mbit with unknown battery backup RAM

DMA: DMA mode for the 2 raw-wave sound channels with timers. 

Serial port speed:  Unknown.  The port pinout is the same, but the usage of it has changed since the GBC.  Expect faster transfer speeds and ease of implimentation for developers

The 512 color mode will allow gamers to experience rich 2-D environments with multiple parallax layers.  In addition, there is hardware sprite and background rotation and scaling in real time.  The 32768 color mode will allow bitmap images to be displayed on screen without action occuring.
 

Check the dmg-ice previews section, or www.gameboy.com for latest information.

The GBA is slated for release on March 21, 2001 in Japan, and a July release is expected in North America and Europe.
 

Front
Back
Top
Bottom

The unit will be available in three colors- Arctic White, Glacier (clear blue) and Grape (old GBC color).

Pricepoint:  $9,800 yen
The MSRP for the GBA is $99 US 
 

The GBA Linkport will be somewhat different from the original GB/GBC.  While it features the same pinouts and will use the same cable, the networking capabilities of the ARM processor is such that different protocols will be used to make 2-player games easier to program and will allow for much faster data transfer.  GB/GBC games will function as normal with the link port.  The IR port has been removed, therefore gamers wishing to play GBC games with this feature will have to buy a separate IR connector.  Further details will be revealed. 

There is also a device in the works that will allow gamers to connect the GBA to the new Nintendo Game Cube system, much the same way in which the GBC was supposed to be able to connect to the N64.  Deatails will be revealed as announced.

The GBA has 4-player capabilities (and indeed Mario Kart Advance has already been shown to have them).
Below are pictures of the standard link cable, and a diagram how they can be connected for multiplayer games.

As has been stated, certain software can allow for multiplayer games with only a single cartridge for up to 4 players using the multiboot properties of the GBA's chipset.

The old GB/GBC games will play in color to the same extent that they would function on a GB/GBC system.  See the GBC FAQ for the details

The GBA has been designed with a fresh new look, and radical new orientation.  The peripherals designed for the old systems may or may not work- but it most definitely will vary between the product.  For instance, peripherals such as the Worm Light will probably still work, whereas the Gameshark probably won't (at least for GBA games). 

Unfortunately, there really isn't much competition left for the GBA at present moment.  Aside from expensive PDA's, and maybe the Bandai Wonderswan, the most realistic competition is from Microsoft, should they decide to go ahead with a portable X-Box type machine.  This is merely speculation.  With SNK out of the loop in North America and Europe, the only portable that will be on the scene is the GBA. 
 

No, it uses a new reflected-color LCD display which allows for less blurred refreshes, more contrasted colors, and very low power consumption.   In other words, no light comes off the screen, and you can not play in the dark.  Buy a wormlight.

See Section 9
 

Nope, although many retialers who take preorders are required to bundle accessories or software with the unit- if you want a bundle like this, then pre-order at any number of online retailers.

Third party peripherals are already on the way- New versions of the wormlight, UFO light, various link cables, lights, magnifiers, and even a TV-tuner are in the works by various companies.