"Ctrl if it's held down and pressed with another key, Escape if it's pressed and released on its own."
It works really nicely once you get used to it.
EDIT:
To do that I first mapped Capslock to Ctrl using the normal Apple Keyboard prefs, then used KeyRemap4MacBook and checked this setting: http://i.imgur.com/99cNs.png
"Ctrl if it's held down and pressed with another key, Escape if it's pressed and released on its own."
It works really nicely once you get used to it.
EDIT:
To do that I first mapped Capslock to Ctrl using the normal Apple Keyboard prefs, then used KeyRemap4MacBook and checked this setting: http://i.imgur.com/99cNs.png
http://pqrs.org/macosx/keyremap4macbook/