In reply to somethingelse:
I don't think I've ever caught a fall belaying in the mitts, however, unless you're planning on catching it on the live rope there should be that much heat transmitted from the rope. I've held my belay plate after long abseils without any problems, which I would guess has a much higher temp, but I may be wrong.
Certainly don't think they'd melt at a lower temperature than that at which your hands feel burnt (have accidentally left them to try on a very hot radiator before with no visible affects), which mine personally don't after catching indoor/trad/sport falls.