In this heat, I really notice my MBP warming up. I decided to check the Energy Saver panel to see which GPU it was using - dynamic switching, apparently, and I could force it to be in high performance (= hot) mode, or let it switch between the two, but not choose the integrated, cooler chip. Boo to that.
So I went here for help:
http://codykrieger.com/gfxCardStatus
Incidentally, under dynamic switching, opening Firefox it switches straight away to discrete/hot, while Safari on the same page stays with the integrated chip. Weird.