I often use the calculator feature built into the Spotlight menu. When doing numerous calculations, I often wish the Clear key on an extended keyboard with a number pad would clear the already-entered text. Alas, 10.6 doesn't enable this. But here's what I discovered:
In 10.4, 10.5 and 10.6, you can use Option-Delete to clear the last word. In both 10.5 and 10.6, Command-Delete will clear all text already entered in the Spotlight menu (as will pressing the Escape key).
Mac OS X Hints
http://hints.macworld.com/article.php?story=20090901181237565