After getting my beloved MacBook Air back from repair yesterday, my Keychain (the place where passwords, security certificates, and the like are stored in OS X) appeared to be terribly corrupted. I couldn't log into any previously-known wireless networks, I couldn't access my MobileMe information, and I couldn't view any stored passwords (via the dreaded "Access to this item is restricted" message). It took me entirely too long to figure this out, but what happened was that my clock had reset to 1/1/2001 after Apple did their repair, and my computer was totally freaking out about it.
So for anyone else out there having keychain issues after your Mac comes back home from surgery, try to just manually change your date to the present day, and hopefully all will be well in the world again. (Or at least you'll be able to get back online without having to tear your house apart trying to find that scrap of paper where you wrote down your 26-character network password three years ago.)