When assigning keyboard shortcuts (on the Keyboard Shortcuts tab in the Keyboard System Preferences panel) for stuff like Exposé (Show All Windows), I always wanted to use shortcuts like Option-Tab or Control-Tab, but these key combinations were not assignable in Mac OS 10.5.
Turns out that they are assignable in Snow Leopard. I assigned Option-Tab to show all program windows (aka Dock Exposé) and Control-Tab to show the Spaces overview. Of course, it's up to you as to what you want those shortcuts to activate.
Please keep in mind that these key combinations may already be used by other applications -- for instance, Control-Tab jumps to the next tab in Safari.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20090903033458661