OS stops receiving left (primary) mouse click

I've got a frustrating problem that seems to have cropped up within the last two months or so.

Infrequently, the left (primary) click stops registering with the OS, so I can't really click on anything. My mouse moves around the screen, I can right (secondary) click, but I just can't perform a primary click.

I'm running 10.6.8 on an Early 2011 MacBook Pro (http://support.apple.com/kb/SP619). The click stops registering both on the trackpad and my Magic Mouse.

A restart solves the problem.

Answer:
Double check that you don't have any stuck keys on your keyboard by testing all the keys, particularly the modifiers (shift, option, etc).
Make sure there aren't any wizards, alerts, or dialog's hidden behind existing windows or in other spaces (this one gets me often, especially running the odd mix of programs I use daily).

Source: http://apple.stackexchange.com/