By accident I found a neat feature in XCode 3 that allows to find the documentation for a string that's under your cursor. Therefore simply hold down the alt key, the mouse pointer changes to a cross and then do double click.
As it's a useful feature more me, I bound the F13 key to this action. To do so, simply open your Xcode Preferences, and under Keybindings > Help > Find selected text in API reference map this action to a keyboard shortcut of your choice.
