Go to the market and find adj brightness, it has an option to set the brightness on the screen to X setting and you can make it so no other app, including the system if your rooted, can turn the backlight to any other brightness. You can lock the phone screen and the backlight will still stay on. Go check it out and see if its the app your looking for?
Edit: Since you said you like to control the brightness manually in this app you can change the brightness from 2-255, toggle the capactive buttons lights on or off, set a custom screen timeout time. The setting for the main reason you want the app is under options in screen brightness.