At the time, I bought a ButterFly Media USB adapter which worked with Win9x, but not Linux. I have patched an existing USB driver to recognize this device - look here for more info. Since then I've also gotten an inexpensive CF PCMCIA adapter which also works well.
Unfortunately, although Linux can read and write the CF cards, the Ampigo only recognizes MP3 files which are written by Win9x. The reason is simple - the DOS FAT filesystem specifies that the EOC (end of cluster) marker is any value of 0xff8-0xfff for FAT12 or 0xfff8-0xffff for FAT16. Linux uses 0xff8 and 0xfff8 respectively, while Win9x uses 0xfff and 0xffff. The Ampigo only recognizes 0xfff and 0xffff!
To get around this, I've written a simple program which scans the FAT of an unmounted DOS filesystem, correcting the EOC markers where necessary and writing the results back to the filesystem.
uncompress/untar it, cd to the directory and compile
tar zxf fattools.tar.gz
cd fattools
make
Then, after writing some MP3 files to a CF card, sync and unmount it. When the unmount process has complete, run 'winfat' on it:
./winfat device
winfat will tell you what it has found and fixed. That's it! Pop the CF card out and try it out in your Ampigo.
Please let me know if these tools for you.