Could people do me a favor and test
ScreenDim 1.05-SO?
It's a 3-day trial version with a slightly different way of doing the dimming which may be more reliable at overriding other things that may be happening on the device. In particular, it might help with a hard-to-reproduce problem that three users (one of them myself!) have noticed where suddenly the screen brightens up for a short time and then goes back to dim. (Unfortunately, it doesn't help with the problem where the screen brightens up seven seconds before timing out. I spent hours working on that problem, and no luck.)