They sure sound like flying squirrels to me! Right size, general coloration and behavior. They can usually be observed with a flashlight if you don't move too fast - they get used to people pretty quickly and they're a joy to have around.

It may still be a chipmunk - they do not officially hibernate - they must come up to eat every few weeks. Also, their coloration in the winter might be more gray than brown.
OR it may be a flying squirrel - how lucky would that be! They live in trees and are nocturnal - take a look at the picture on enature -

That's it! Many thanks for your help. The photo from enature.com was inconclusive, but thanks to your suggestions I searched for other flying squirrel photos and have made a positive id.

This also explains an incident a couple of winters back when I was standing at the window at night (with the flood light on) watching the snow come down. Something "flew" by the window, landed on a hemlock trunk, and quickly ran up the tree. My immediate reaction was that of a bat because of the flight at night, but, again, bats are supposed to hibernate I thought, and running up the tree didn't sound characteristic. We've obviously had these flying squirrels around for years without know it - how exciting! Thanks again for your help.