thanks to everyone for the help!

the good news is, the fix was free. the bad news, it still cost me $18 for the tow when i could have fixed it at the gas station! it turns out that the ground wire on the bottom of the alternator had fallen off. had i looked around a little more, i probably would have seen that. i put it back on, the voltage across the battery terminals is 13-14V when the engine is running, and all is well. so the POS brand alternator rebuild is going back to Advance, and all i have to do is take off my hubcaps before those monkeys at sears break them while putting on my snow tires tomorrow (7am. . .before the mobs hit the mall!)

i guess this just reinforces the lesson that (with these cars, at least) the problem usually has a simple solution, but requires some complex digging to find it