Apple's Developer Tech Note TN2056 explains how you can create your own keyboard layout. I've used it to remap the "plus minus/paragraph" key (the one above the Tab) so it creates a backtick and a tilde (when shifted), just like a PC keyboard.
[Editor's note: My backtick/tilde is above the tab, but I have an Apple standard keyboard. The technote is very detailed and will be quite helpful if you're interested in setting up your own customized keyboard layout.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20020917052722716