Hello. I have noticed the same problem with certain .mp3 files and I can not seem to figure this one out. I get a lot of my music by ripping a stream to my computer (mac) using a ripper and then just transfer the files to my Curve 8310. I will find that with certain songs (no pattern) I will also get the "unsupported media" error in the BB Media Player. The interesting thing is that these same files will play in iTunes and also my iPod.
I have been trying to transition from my iPod to only the Curve since it is so much more convenient to carry only one device... but in my opinion the media player from Blackberry looks great but is immature in functionality and efficiency.