Check this Thread about reverting back to 7.2 (ONLY if you're currently NOT using the iPhone).
http://discussions.apple.com/thread.jspa?threadID=1017261&tstart=0
As this Album Cover issue relates to AIFF tracks and if you're not yet using the iPhone, I found that reverting back to 7.2 recovers the viewing of Album Art that existed prior to the 7.3 install.
In the Thread referenced by the Link above, I've provided instructions about reverting back to 7.2.
MacFixit also has posted instructions, which vary from mine in that they recommend deleting the iTunes .plist file. Since I didn't want to lose any preference settings, that can be contained in a .plist file, I didn't want to delete that file.
To date, I've found that my reversion method has effectively put my iTunes back to same state as prior to installing 7.3 through S/U.
I'm happy and await the next update to 7.3 to see if it corrects this issue with Album Art and AIFF tracks.