I just jailbroke and unlocked my friends iphone which was already on 2.2.
He installed flashlight, idrummer, five icon dock and winterboard and then a few minutes later, his backlight is completely off. We've tried changing the brightness in the settings and still nothing.
It's not like the brightness is turned down though, it's fully off.
Do you have SBsettings installed? I found a bug between Flashlight and SBsettings. When you close the flashlight app it makes the SBsettings brightness toggle go to zero. You just have to open SBsettings and move the brightness up. Then, lock and unlock and it's back until you open Flashlight again.