Luckily, it got caught quick and I'm not out anything.
Somebody cloned my damn credit card and started using on Wednesday. Thanks to Discover for catching it so quickly; they called me within 6 hours. I guess their NASA supercomputers realized that I would never buy (and have never bought) gas and McDonald's in the post-lunch-hour afternoon of a workday in the middle of the week.
I've got no idea how my info got loose, but Discover's cool about it. I'm not liable, and they're expressing me a new card.
Now I just need to know which merchant has crap security.