Originally Posted by Nos4r2
No need to worry.
You do need to be certain that the lower unit drains completely after you haul out. I always lower the engine right down once the boat is on the trailer. If it's below freezing, I'll turn the engine over for a couple of seconds to blow out any water around the impellor. You also want to be certain that there isn't any water in the gear lube. I've seen cracked lower units as a result of this.
Of course all of this is based on fresh water operation. I suppose you salty types get a few degrees of leeway!
I've used my boat in sub-zero temps for years and never had a problem...