Ever since installing Tiger, my PowerBook has been scorchingly hot. I often run Marcel Bresink's Temperature Monitor to keep an eye on the internal temperature of my notebook and had never seen numbers so high as those I saw after installing Tiger (and by "high" I'm referring to temps north of 145° F). Consequently, my fan had never been on as much either, which was incredibly annoying.
That said, I seemed to have fixed the problem by resetting both my PRAM and NVRAM. If you're experiencing the same heat issues, you might want to read Apple's How to Reset PRAM and NVRAM and free yourself from the oven.