I have had exactly the same problem with the dimmer as the others describe here. Sometimes when I get in the car (lights off) the display comes on 100% brightness. Other times it seems to be at about 80% brightness. If I turn the car lights on and off it goes to 100%, but only sometimes. It seems random. If I go to the setting via the menu, it will show 100% when it's not, going down to 90% and back to 100% normally fixes it too, and I always use the buttons not the wheel.

I've not had time to investigate further, but It's only happened since:

I got a new 60GB model (replacing my 10GB which I sold)
Beta 7
I also run Hijack now (various versions) so it could be that?

I hope its Beta 7, and not a hardware fault, but I'm starting to wonder!