Not sure if this has been posted, so here goes...
The PSP doesn't support iTunes AAC straight off the bat, so here's how to get them to run. If anyone's got another method, let us know.
For anyone else wants to use iTunes compressed AAC tracks on their PSP there's an easy (and relatively quick) way to transcode them without having to recompress them. If you open Quicktime 7 and Export as .mp4 (has to be registered) then choose the passthrough option in the encoding the file just gets transmogrified into a PSP compatible AAC track. It's quite quick because no re-encoding takes place.
A pain in the ass if you have an albums worth, but there's an Automator action to do a bunch of files one after the other (using the last settings in QT Export dialogs) here...
http://www.automatorworld.com/?p=143
I'm on 10.3.9 here at work, so can't verify the exact steps, I did it at home on my powerbook last night.
mud