I didn't panic, because I have been keeping backups. Especially with photos - since returning from Japan I've been just buying new memory cards instead of formatting and re-using them. Plus most of my photos that are any good are already on Flickr at their full resolution anyway.
Now I needed a program that wasn't payware or crippleware so that I could get my files back. I found a free, legit utility to undelete files - though in this case the files weren't "deleted," but I think what happened is the Master File Table went bad and "forgot" most of the resources on the drive. The program works just as well for forgotten files as it does for deleted ones. Actually, deleting is just forgetting. Deleting a file used to amount to just striking its entry off the table, but not actually doing anything with the bits. Now it means "moving" the file to the Recycling Bin folder, and when you delete from there it's struck off the table - but still, nothing's done with the bits.
So that's the good news - the bad news is that when you write new files to or modify existing ones on that partition, there's a chance that the new bits will just happen to chew up your "invisible" old bits. So a prompt recovery operation is key - not only that, the program authors advise recovering to other partitions / drives for these kinds of operations. Good advice, because that way the recovered files won't overwrite or chew up your yet-to-be recovered files.
The recovery is going quite well - it's even recovering stuff I actually didn't want to recover, like discarded exposures from bracketing. So when you do something like this, try to make sure you don't ask it to restore any folders named "RECYCLED," as that's basically your Recycling Bin for that drive - nice to know you can recover from there just the same as anywhere else, though.