It's the regular wired Mac keyboard, and the other is a wired Adesso for Mac. I'd already downloaded that Mini '06 keyboard update, but when I opened it I got an error message that it couldn't be installed on this machine.
It's only the left shift key that doesn't work, there are left and right shift keys on both keyboards. On the Mac keyboard, the left shift key doesn't work at all, and on the Adesso, it comes and goes. Very strange.
I've changed USB ports, and made sure that the left shift key is not a hot key for anything. I ran disk utility and there weren't any permissions listed about a keyboard.
My system has been running very slowly lately and I'm terrified that this is a sign of something really wrong. I called Apple and I'm out of warranty, they said I'm SOL and that I should check their support site and try to "troubleshoot and isolate the issue."