I recently had a problem where the keys on my MacBook Pro keyboard did not work. The modifier keys and arrow keys worked, but the letters did not. Using an external keyboard worked fine.
It turned out that the problem arose from accidentally enabling mouse keys. It seems I had somehow pressed the Option key five times in a row. To fix, press the Option key five times in a row again, or go to the Universal Access System Preferences panel and disable Mouse Keys. You might also want to disable the check box to enable Mouse Keys by pressing Option five times.
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Mac OS X Hints
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