For short stops like that I carry a thin, lightweight cheapo lock. I don't have very valuable bikes, and I figure anyone who might pinch a bike on a whim like that is not going to be carrying a humongous lock cutter around. It's not hard to cut a lock, but you can't do it with standard home tools, at least not the ones I have.

Bikes that are regularly locked up at the same place and time, or left in a spot with many other expensive bikes are definitely at risk from the professional thieving lot though.