Tracks purchased from the iTunes music store are protected AAC and obviously won't work with rhythmbox. However, when you try to import those songs, rhythmbox takes forever to attempt to import them. RB should probably find some way to identify the files as unplayable/unimportable and just fail to import them rather than taking 10 years to do so. Note that the songs are shared over an SMB share, so that might be why RB was taking a fairly long time to look at the songs. I assume that GST is reading the entire song file looking for file magic or something it ID the file type with?
RB also fails with the same symptoms to import m4a content, which is AAC-encoded audio in an MPEG4 file thats NOT protected content, ie stuff that it could actually play if GST could decode it.
I don't have any files to test this with. If this is still an issue in FC6/FC7, please reopen for those products.
If you install the right versions of various codec libraries, you can actually play m4p and m4a content, which I just figured out :) That's really neat. It's probably harder than its worth to try to detect which gstreamer plugins are installed and what they can play from inside rhythmbox when importing.