So I took my blackberry for a walk during a very frosty night... an hour or so later I want to check the time... and my screen is messed up
A whole bunch of pixels towards the top of the screen died, and all the pixels underneath them (about 80% of the screen) displays nothing but white color. Surprisingly the upper 10% works just fine. What happened? Is this common?
No, I doubt any water got to the unit. It was not raining, but it was lightly snowing. The berry was in a bag (thus, not against my body where it could get some heat) ... the unit was freezing cold when I took it out of the bag, but c'mon isn't technology meant to stand winter days?
Could be condensation forming on/inside a very cold phone when you took it back into a warm (moist) environment. Allow it to warm up at room temperature, probably overnight to allow moisture to clear and check again.
LCD displays and batteries also don't like to work outside their rated temperature range, which probably doesn't include below freezing. Think about it... LIQUID crystal, and the batteries are a chemical reaction which is sensitive to temperature, and have a liquid electrolyte (bound but still liquid or the electrons can't move).
If its still broken in the morning... its broken, and fortunately its Monday so you should be able to arrange repair or replacement.