That really doesn't sound right... A screensaver shouldn't stress the machine anywhere near as much as to lead to those kinds of temperatures. When I let my machine sit on it's own til the screensaver goes on (which usually only happens when I'm having the machine play music when I'm reading a book, doing university course assignments on paper or cleaning) it doesn't get any hotter than it's typical idle temperatures of around 40-50c.
When I think about it, the only logical thing I can think of is some kind of background processing (like Finder indexing, Folding@Home, etc.) combined with ether clogged up or otherwise malfunctioning fans.