A very generous passerby helped me get it home, and the next day, my housemate gave me, the bike, and the wheel a ride to Via Bicycle, where it was established that the wheel was from a different bicycle. A different-sized bicycle. Mine has 24" wheels. This was a 27" wheel.
It wasn't that some irresponsible person had a flat, either.
Somewhere in Philadelphia, a partial bicycle thief is riding on a slant. On a French bike.
Everyone I've told this story to thinks it was worth livejournaling, so here it is. I didn't think it was all that interesting, so anyone who's entertained owes thanks to several of my friends.