Winter Cycling - Freezing locks?

It was in the low 30s (F) yesterday and my ULock came off fine. After about a mile and a half ride I had to make a stop and was unable to get the lock open--the key would only turn half way. The lock was safe from the elements (it was snowing) and has worked fine beforehand, whether dry, wet, or 20 degrees. Chances are my cold fingers erred in closing it properly and screwed something up, seeing as the lock still doesn't work after being in room temperature for a while. Either way, I'm still curious about what effects (if any) the cold weather has on locks and what experiences others have had. I'm using an Onguard ULock and I don't want to spend more cash to encounter the same problem, especially if my bike is locked at the time! Luckily, I had a cable lock as back-up, which may be all I need in the winter anyhow.... Are some locks more winter-friendly, or are all locks created equal in that regard? Thoughts?

Thanks.

P.S. I found this (http://bikeforums.net/showthread.php?t=44942) topic, but figured I'd post anew since it's over 2 years old.

You can use those little spray bottles of de-icer that they sell for car locks.

You could cover the lock itself with something to keep water out. I don't know what. Maybe a condom?

How about using a little bit of graphite or silicone lube as a preventative. Something not too greasy and just enough to work internally. I wonder....would the chain wax lubes work? They don't seem to freeze up on the chains.

Just had a Kyrptonite cable lock freeze up on me. I think it had a little bit of snow inside of it before I closed it. I just held it in my hand for a few minutes, and it loosened right up. I'll have to try some of the silicone lube next time.