I had the same problem and it was doubled by having auto lock turned on. I got tired of unlocking every 30 seconds. I turned it off, although I've had a few appointments called "Sssssss" since then from bouncing on my belt!
I got the "StopDim" applet (
http://gotreosoftware.com) and it solved the auto off problem - even after a phone call or hotsync, the setting I chose stayed (up to 3 min).
Cingular's new firmware upgrade was supposed to fix this bug (Palm called it a "feature" - I never tire of laughing about that one). It seems to have done the trick. So now I don't think I need StopDim.