Alright, I just talked to them. They said that the overflow cap was also faulty, and replaced it free of charge. They also said that there is a slight head gasket leak (near the back of the engine) that is contributing to my minor oil loss (about 1 quart every 1000 miles), coolant loss, and elevated temperatures in traffic. They said it wasn't completely "blown", and that I could have it replaced when time and finances permitted. Does this make sense?

I wonder whether the new radiator was necessary. I will question them thoroughly, and take it to a good independent next time.

*EDIT* I know the thermostat is good because it was replaced last week; the water pump was replaced within the last year or two, and I would be very surprised if it was the problem. I don't think it is cost effective just to replace it if we have no reason to believe it has failed.