Mac: What's the keyboard shortcut for switching between video mirroring and extended desktop screens?

It's ⌘ - F1 or ⌘ - fn - 1, depending on the setting according to Chealion.

I found this on Mac OS X keyboard shortcuts. It's listed there as "Command-Function-1," but on my current generation (3rd generation?), unibody, MBP, ⌘ - F1 is what works for me.

On older laptops (eg. earlier MacBooks, MacBook Pros, Powerbooks, etc.), by default it's simply F7. You will need to press Fn if you have "Use F1, F2, etc. keys as standard function keys" checked off in the Keyboard and Mouse Preference Pane.

⌘ - F1 is supposed to work but I've found it quite spotty in terms of whether it worked or not on desktop (eg. iMac, Mac Pro) machines.