There has been concern in recent weeks about the accessibility of iTunes, so Apple has released a PDF statement at
http://images.apple.com/support/itunes_u/docs/iTunes_U_and_Accessibility...
The big news is that iTunes 7.1 (March 2008) now includes VoiceOver compatibility which should allow most screen readers to access the iTunes menus/navigation. I haven't tested this, so I can't verify functionality.