Well, I am using a profile application called phoneAlarm.
With that you can set up many phone features to a profile. Among them the phone back light level.
It also enables you to switch profile based on time. Thus, I have a home, work and night profiles that are automatically activated based on time and control my phone volume and back light level among a lot of other phone settings (e.g. wifi, bluetooth on/off).
HTC Desire + Jabra BT-8010History:
Android Version: 2.2
Kernal: 22.214.171.124-froyo_bravo_V5 root@Alhena #9
Build: 2.29.405.5 Droidzone
ROM: Starburst 126.96.36.199
Stock Rom: 2.2
build number: 2.09.405.8