Recently a friend shared a beer with his Macbook Pro, which resulted in a dead Down Arrow key. One way to work around such a problem is to enable Keyboard Viewer (System Preferences » International » Input Menu) and use it to click the dead key instead.
The viewer can be toggled on/off from the menu bar if Show input menu in menu bar is also checked. Apparently Apple charges quite a lot for a repair involving a liquid spill, so if the broken key is not a frequently-used alphabet or punctuation key, perhaps this is a viable solution for users in this situation. It isn't the keyboard viewer's intended purpose but it works.
[robg adds: If you were going to do this for any length of time, you might also want to use this hint, which lets you toggle the viewer off and on via an AppleScript.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=2009042314081710