Guys - Got the problem resolved by running hot water on the door at the car wash and then gently prying the door open while chipping out a small ice rink that had formed in the filler opening. I'm guessing the black rubber grommet at the lower left hand side of the gas door is the drain plug, and it was frozen so water/ice couldn't properly drain out.

Thanks to you all for the helpful and quick responses. This is a great forum, and you guys are the best.

The message is if you get your X-Fire washed and the temps are in the single digits, make sure you park in a warm garage (and have a full tank of gas!).