I have the experience of shorcuts not working on Panther's Finder after some time working in Finder or other apps, but this only happens with the shorcuts of the File menu, I noticed that if I just click on the File menu (not a command, just the menu itself) the shorcuts work again. I thought it happens just to me, but recntlty I visited Apple discussions and found that some other had the same experience.
Hope this remedy work for some of you instead of rebooting your Mac.
Mac OS X Hints
http://hints.macworld.com/article.php?story=2003112712404723