Tasker should be able to do this, but it's only in beta and not available in the market yet.
You could setup a Tasker Profile Event for Phone Ringing to perform an action to set the Display Screen Brightness to whatever you want. You would also have to add a Wait action for about 30 seconds or so, and add another action to reset the Display Screen Brightness back to your favorite value. It's too bad Tasker doesn't let you do Exit tasks on Event Profiles since it does have exit task on other types of Profiles.