There seem to be a lot of rules about what keys combinations you can and cannot edit in the Keyboard preference pane. Apple has definitely forbidden the creation of simple command-key shortcuts, for instance. Also, there are other limitations out there about replacing existing shortcuts as well. There is a simple way around these limitations however. Open up the file ~/Library -> Preferences -> .GlobalPreferences.plist with Property List Editor. Fnd and open up the NSUserKeyEquivalents key. Edit away as many keyboard shorcuts as you like, without Apple sitting on your shoulder and telling you which you can and cannot use. You'll need to know the modifier symbols:
What I have used this hint for is to stop my browsers from quitting on command-Q (they are now all set to command-control-Q), as I use command-tab to flip between my HTML editor and browsers all day long, and had grown tired of accidentally hitting command-Q and losing all my open files or web pages.
You do have to replace the command-Q shortcut on an application-by-application basis, but it doesn't take long to duplicate the command and paste.
You can create dummy keys for each application in the GUI Keyboard Preference pane which makes the Property List Edit go faster.
There's no need to work on each application's own plist files; all menus are fixable in globalpreferences.plist (I had some trouble with Adobe applications, though). Don't forget to quit and relaunch applications to see their new keyboard commands show up (visible in the menu).
[robg adds: This is discussed, in less detail, in this previous hint.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20040322141804767