Electronic devices usually should be kept at room temperature. I don't know if leaving it plugged in will work. I have noticed some electronics do get warmer when plugged in, but I am not sure this will be the case with your phone. I would definitely not use the phone right away when I got back in the car. Instead I would try to wait until the car heats up and the phone feels warm again. This way you avoid operating the phone under any kind of stress from the severe weather.

 

I'm thinking the OP meant 30 F, because 30C is actually quite warm. As far as the device goes, your main concerns are the battery and the screen. The battery will not work as well in cold temperatures; its chemical reaction is slowed by lower temperatures. The phone will likely show that the battery has less charge than it actually does.

The screen is another story. You may notice that it "ghosts" or shows some burn-in if you try to use it while it is that cold. This isn't permanent damage. Simple fix is to warm it up in your pocket for a few mins before you start using it.

Since you're not using the phone for a few hours, if you want to be on the absolute safe side, just turn the phone off while in the cold (or heat) and turn it back on when at a more normal temperature.

 

Here is a thread from me from last winter ...

http://androidforums.com/lounge/3164-g1-cold-weather.html

There is mention from me about being on a frozen lake. I forgot to post up about that.

However the phone was in my belt holster for 6 hours, while standing on a frozen lake. It was about +20 that day so not very cold.

 

Personally I'd be more concerned about taking the cold phone into a warm humid building and having condensation form on it. I'd GUESS that doing that often enough could cause a problem. I'd warm it in a pocket first, or something like that.