Thanks for all of the good information! I'm still fighting with the stupid USB trying to get a 2GB flash drive and a couple dozen MP3s to work. I've tried every combination of format and cluster size, and no luck so far.
To whoever mentioned NTFS, I wasn't able to get the system to recognize NTFS no matter what I tried. Using Fat32 with various combinations of cluster size I was at least able to get it to tell me no supported data found, so I'm thinking NTFS is out (unless there was a firmware update on the radio I'm missing).
Tried eliminating all variables from the filenames and still nothing (I have 2 folders now, labeled "1" and "2" with songs named "1.mp3" through "12.mp3" in each, and still nothing.
I was wondering if the fact that they are 320kbps may have mattered, but sounds like at least some people were able to get that working.
Good info on the ID3 tags, I'll have to check on that and see if it makes a difference.
I get the feeling there may also be some variations in the radio firmware. I know there was at least one TSB that involved replacing the PDIM module and flashing the firmware. The instructions on the TSB to check the software version don't want to work on mine, so I'm thinking a trip to the dealer is in my near future.
What a pain...