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You were able to in OS 9 if you enabled multiple users, but it never really worked right. I don't know if you're able to in OS X. It might be a feature in the Universal Access preference pane...

Edit: I just checked the Universal Access pane. I didn't see anything. I don't think this feature is built into the OS anymore.
 
this idea completely negates the idea of a password. if you're sitting at your computer saying your secret password aloud, guess what? it's not a secret anymore.
 
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