IGN
 
The SongPro- The world's first module MP3 player
Status: Unavailable, may never be released
Ever since it's announcement, this unit has been the buzz in the GB community.  So here's the skinny on the product that every music-loving GB owner wants to get their hands on. 

Technical details:

A portable MP3-playing module which supports 2 formats:

.MP3 - the world's most popular streaming audio format,
.MPE - Destiny's encrypted MP3 format which contains visual information to accompany the music, such as album covers, lyric sheets, etc.
.WMA, .AAC, .AC3, .VRC, .WAV, and SDMI formats are all supported

The unit will come with 32 MB of space on it- enough for 30 minutes of stereo 44khz 160kbps music.  Additional 32 MB modules will be released separately, the fomat is confirmed to be the tiny and expensive MMC format.

link:  www.songpro.com

After a hefty legal battle with Nintendo, it appears as if there has been some form of resolution ending in favor of the SongPro team.

Songpro's website has been updated to list a number of SongPro products- 2 of which are slated for release on the Gameboy/Gameboy Color/Gameboy Advance.  The first of the 2 units is the "SongPro for Gameboy" which features onscreen effects or lyrics depending on the format, has a USB connection, 32 MB of built in MMC memory (with 2 MMC ports), an external mic input, and standard headphone jacks.

The Songpro Junior features no internal memory and only one MMC port, and still features the same hardware and software buttons as the above unit.  Otherwise the features are pretty much the same, the Jr. features a USB connection, external headphones jack  and external microphone jack.

I've spoken a couple of times to the guy who runs SongPro, and unfortunately he doesn't share the same vision as his predecessor, so I don't think that I'll be able to receive a trial unit or a review copy, so you'll just have to wait until it hits market until I can get to one to take a look at...