The screen dims about 30 seconds before the time you have autolock set to (ie if you have 5 minute autolock the screen will dim at 4 min 30 seconds) to stop this just turn autolock off.
Remember, even with autolock on "never" if you manually turn the phone off and a button gets bumped it will still only stay on for 10 seconds before going back off, so you don't run the risk of the screen being bumped on forever once you manually turn it off after use.