I have all of my music stored as AAC lossless on my computer. When I tried transferring some songs on a thumb drive to my PS3 they wouldn't work. I did find a link to a workaround to do it, but it seems like a pain to go through so I haven't tried yet.

http://www.tomkiss.net/blog/how_to_import_an_itunes_playlist_to_your_playstation_3

This was all about a year ago now and I haven't messed with it since, but I sure wish it were easier to get my music on the PS3.