Correct me if I'm wrong, but can't you just hit the Power button on a laptop to bring up the "Are you sure you want to shut down your computer now?" dialog? Once that's up, you can switch to any application away from iChat. And all without disturbing the user on the other end. Once you're in another app, you can dismiss the shut down dialog. I could be misremembering, though.

Yes, or if you use Spaces, you can use the keyboard shortcuts to change to another Space.

Here is an Apple discussion board message about using the power button on a Mac portable to regain control of your Mac without moving the mouse. This doesn't work for my Mac mini.

http://discussions.apple.com/thread.jspa?messageID=11607637