I just learned about this today from a dude I sold a bike rack to on CL. After he told me about it I came home and looked it up. You don't even need liquid nitrogen. It's as easy as keyboard blower and a hammer. The idea is that when used accordingly the keyboard cleaner canister can produce a very cold spray (similar to a quickly spent co2 cartridge just much colder) that can be used to freeze the lock, then you simply take a hammer to it. The sub-zero temps make the steel in the lock more brittle, then the impact of the hammer does the rest. From the videos I've seen, it's like the lock is made out of glass and it take less than a minute to do. I'm not trying to train thieves here...just educating fellow cyclists about what we're up against.

Although, all the demonstration videos I've seen show the lock being hammered against a hard surface i.e. concrete or a steel table. But I have a hard time seeing this method work on a NY Fahgetaboutit when it's on the bike and up off the ground. If you took a hammer to a lock in that position it wouldn't absorb the full impact and would just bounce out of the way (at least I hope).