I finally bought a Creative Labs Muvo 4GB MP3 Player, took it apart, and ripped out the Hitachi 4GB Microdrive nestled inside. This is a FANTASTIC deal! Hitachi 4GB drives are around $500 purchased separately, and the Muvo 2 is only $199. J&R had them in stock last week.
I did run into one hitch, however. When I took the drive out, I immediately stuck the thing in my Canon 1Ds and formatted it, resulting in a 2GB FAT partition and 2GB of unused space. Those of you who are familiar with Windows XP know that it is impossible to repartition CF media using the operating system, but luckily, there are two Powerbooks in the house. OS X was able to repartition the drive, and now have a huge 3.81GB microdrive working in my camera, at full capacity.
Note that it is possible to format the drive to 4GB FAT by using XP, Linux, and some trickery. But since my camera supports FAT32, I didn't have to resort to those methods.
UPDATE - it appears that my 1Ds doesn't work with the Hitachi 4GB drive. I tested it on Elliot's Digital Rebel, and it works just fine (with the full 3.81GB available)! I'm going to have to figure this out. :(
UPDATE 2 - here's a link at robgalbraith's site that talks about the fix. I ended up using Elliot's digital rebel to do the actual formatting, and now it works!