It depends on how the MP3 player "reports" itself to the USB interface. If that connection is pretty standard and straightforward, it will work.
If your MP3 player requires a special driver to work on Windows for example, chances are 100% that it also does not work on the BMW USB interface. As far as I understood it, this is why some USB harddisks work and some don't. All USB memory keys work and indeed some MP3 players.
(My guess is the ones that work plug & play on Windows), and of course the IPOD, as BMW made sure this one worked as it's so popular.
I guess when this USB thing becomes more popular in all BMW's and people start to get rid of the CD-Changers and loaders, we will hopefully see BMW start supporting more types of connections and publish a list of supported players.
(would love to see them support the Zune..)