You can now tab your way through open/save dialog boxes to select cancel, or whatever. Tabbing gives you a highlight surrounding each button you tab to. The trick is to hit the space bar to select!
I don't believe this was a part of 10.1 (hate to think I missed it all this time if it was), but it is a godsend in Jaguar.
[Editor's note: In testing this hint, at first I had no luck with the tab key. It simply selected the input box and the navigation box and nothing else. So I dug around a bit in the System Prefs, and found that I had to change a setting in Keyboard -> Full Keyboard Access to make this work. At the bottom of the pane is a "For windows and dialogs, highlight:" section. Click on the "Any control" radio button, and tab and space will work as described. I'm not sure if the author had already done this, or if our two machines are behaving differently, but it's clearly a great timesaver!]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20020829053723368