I have found that my truck warms up much faster on a below freezing morning when i leave it idle for 5-10 minutes in drive. This raises the egt (from 200* when idleing in netural) to about 500* when idleing in drive or reverse. This also alows the trans to warm up much quicker. I mainly do this when I do not have access to power for the block heater. My question is, aside from the ovious danger of the parking brake failing and the truck driving off without me(also wheels are chalked), is there any reason that I should not be doing this?