It was officially corrupted data, so I had to erase my hard drive. Boo.
But thanks to Super Duper, I was able to get back to where I was on Thursday when my last automatic backup ran. And it took about 3 seconds to redownload the email I’d received since then, and I hadn’t done anything else on my computer since then. Yay!
Although I did come perilously close to buying a new MacBook Pro while waiting for the guy at the Genius Bar to confirm that it was just software and not hardware causing the problem.
I do hope they were able to clean up all the drool after I left.