Replying to Canon T1i owners: using on-camera flash manually?

Please help me out. I've just gotten a Rebel T1i and have gotten it dialed-- except for one thing.

Is it possible to take the on-camera flash off the E-TTL II mode, and control its power manually? I see the box in the menu, but it says "E-TTL II" and is grey, AKA I can't change it. I need to be able to stop down the on camera flash manually, ie choose M, 1/2, 1/4, etc.

This is because I often use my external flash as an optical slave and the on-camera flash as a downpowered fill, so I want to be able to control exactly how bright it is.

It would be downright stupid if this weren't an option, I just can't seem to figure out how/where it is.

Anybody have ideas?