I installed Snow Leopard three weeks ago and have now started to notice the same problem. The computer will be off all day and as soon as I turn it on, it heats up in no time...I just turned it on after it sleeping for a few hours and it jumped to 158 within minutes, even though I'm only running Safari, iTunes, Skype, and Stickies. I am not using it any differently than I did two or three weeks ago. I can't offer any solution but wanted to let you know that there are others experiencing the same thing, and hopefully if enough of us call Apple the problem will be solved.
Just upgraded my MBP to Snow Leopard and noticed right away how quick and more efficient it felt. My macbook would heat up easily in leopard and run full blast even with the lowest performance setting. Apple had an update then that sort of helped fan speeds to decrease, but now watching hulu alone makes fanspeeds reach 5500rpm(using istat widget) and maintain almost around there during streaming then slows down after being paused for a little bit. The aluminum case is always hot after some use and with snow leopard there's no settings for performance. Is this normal for my MBP to work so hard just to do these things? I have windows laptops older than this that still don't run so hard while doing equivalent tasks.