I've had the game for a few months on PC and the game always worked fine with the 360 controller.  A few days ago I decided that I should have both my gamepads (on 360 gamepad, one Saitek) plugged into my PC all the time, because it makes it easier to enjoy games that work better with a controller.
 
Only problem is Mirror's Edge doesn't want to work with my 360 controller if the Saitek is plugged in at the same time.  There are no in-game options to switch main controller AFAIK, and changing the priority in controller settings in Vista doesn't change anything.
 
Is there any way to fix this?  I don't need the 360 controller emulator because I'm using a 360 controller!  Mirror's Edge is the only game misbehaving like this right now, other Games for Windows Live games are detecting the 360 gamepad just fine...

I don't think there is a way. I get this problem in Prototype with my (non MS driver) 360 Hori stick plugged in. It seems to want to take the directinput controller over the xinput controller.