Same thing happened to me the other day. Thought I was being real smart by opening the trunk and crawling through to open the drivers door. Go around, jump in, try and start the car. Zip, nada. Flashing hazards clued me in to the fact that the immobilizer was still on. Duh!
Of course you can only turn this off by unlocking one of the front doors. Fortunately, by the time I figured this out, the key had warmed up enough to turn the lock. My first stop of the day was to buy some of that lock de-icer stuff. Haven't had to use it yet, so I hope it works.
Matt