I was trying to write an e-mail this morning and Mail wasn't filling in by name. After much fussing, I opened the address book... and it was empty!
I have syncing turned on on 4 machines, so I'm not sure which one of them lost its tiny little mind, but the rot hadn't extended all the way: there was still one machine with all of the data, so I quickly reset .Mac sync from that machine, telling it to overwrite .Mac with the data from that machine. So everything's back to normal, but in the confusion, I think I did lose one of my calendar entries. Who knows what else has gone bonkers. Sheesh.