I was adding a .txt extension to a file without any extension, and got the usual dialog asking to confirm with buttons "Add" and "Don't Add." By intuitive accident, I hit Command-A and was the suprised when it actually worked.
When changing an extension, however, the dialog that is displayed instead has the buttons "Use .txt" and "Don't Use .txt." I tried Command-U and a whole lot of other key combos before landing on Command-Clear (located above 7 on the numeric keypad). This combination seems to select the non-default button in both the "Add" and "Use" versions of this dialog (and maybe others?). I'm running 10.4.2, and so I don't know if this works in earlier versions.
[robg adds: As noted here in a previous hint, you can also use Tab to select and Space to activate any of the buttons in such dialogs...]
Mac OS X Hints
http://hints.macworld.com/article.php?story=200510181658193