Modifications made to the engine, transaxle, exhaust or fuel system of your vehicle, or the replacement of the original tires with other than those of the same Tire Performance criteria (TPC) can affect your vehicle's emission controls and may cause the "Service Engine Soon" light to come on.
Obviously, I just changed a tire -- it's the same kind as the other three, but there are differences in wear patterns, and I don't know if they checked and filled the pressure in the old ones, so there might be enough difference there to freak the chip out.
Rewiring the fuel pump might also count as "modifications to the fuel system".
halfelf revealed that AutoZone stores will check the OBD II chip for free, so, once I grab a shower, I'll head out to the closest one.
[[Edit: Crap. No, they don't, anymore—at least not in California. Argh.]]
I didn't mention the odd not-quite-metallic smell that I noticed when I got out of the car last night; thoughtsdriftby confirmed it. If it was coming from my car, then it might still be a bit more complicated than New Tires.