How to set Autobrightness
Since some folks requested it, I thought I'd explain what I did:
The autobrightness settings are under Settings->OpenDesire Settings->User Interface->Automatic Backlight.
"Light sensor filter" -> Disabled
Under "Light levels":
-> Use custom -> Enabled
-> Screen dim level -> Set to 10
-> Allow light decrease -> Enabled
Now for the fun part, the settings under "Edit other levels...":
At the top of that screen is the light level currently being reported by the sensor (the raw one). The light sensor is at the top of the screen underneath the black bezel, just to the left of the HTC logo. Move you hand over it to make the light change and see what readings you get. With my phone, my sensor reports only 7 discreet values. Those values (for me) are 10, 225, 320, 640, 1280, 2600 and 10240.
So I set the number of levels to 7 and used those values as the "Lower" boundaries. Next, I set the screen brightness I want for each of those levels. I used increments of roughly 45 to give fairly even steps. Notice also that I set "buttons" to 0 for all levels so that the key backlight is always off.
That's it, in a nutshell.
"Those who mind don't matter and those who matter don't mind." ~Dr. Seuss
Running CM10.1 on a Samsung Galaxy S3 (SGH-I747M - d2att)