Further updates: I checked esxtop, and saw that all CPU cores were idle. CPU temp jumps from 66 degrees to 98 degrees as soon as the loadscreen flips from loading the modules to initialising VMkernel.

And now for the interesting part: I forgot to shut down at the end of the last boot, and went for lunch. On returning from lunch I saw that the CPU temps were more or less normal, so it seems that something very strange is happening - it was at 98 degrees for at least an hour before I went to lunch, so although esxtop shows no load, the CPU is still kicking out a lot of heat. Time to grab something to graph the temperature, I think.