Are you running it off a stable power source?
Yep, running off a 7.2V battery, which checks out at about 7.4V with the meter. The battery feeds a typical 5V power supply (7805), and I'm measuring 5.1V out of that supply onto the rail that powers the atmega.
Edit: Figured it out. On a side note: Why do I always have to be the person folks point to and say, "don't do it like that". Geez...
For the record, I was trying to use the UART before it was initialized. In my defense, the code is in C++ and the offending statement was in a class constructor that was called before the UART was initialized. It wasn't completely obvious considering both the UART init statement and the rogue printf statement were in different files, even.
Hehe, live and learn. Thanks for the moral support, guys (and the occasional gal).