I have my PPC Mac Mini to sleep after an hour of inactivity. Well for the past couple of months it hasnt. I'm running the latest updates with Tiger but I just dunno what is causing the problem.
OK, try loading Activity Monitor (Applications/Utilities/Activity Monitor), without doing anything else, and see what the CPU usage is. I'm thinking a background process could be running and using all your CPU so it isn't going to sleep.
On a similar note, did you install any software when this started, and have you installed any haxies? (e.g monitor spanning hack, Uno)
Idle in the 90's sounds like it should be sleeping, an OS reinstall would probably solve it, (Archive and Install) but this is probably like using a hammer to squash an ant.