I have also stumbled upon a local bank that is using unsecure wireless, and informed them thru before written letter and via email. That was several months ago, yet as far as I can tell, nothing has changed.
I can sit in the shopping center parking lot, or for more comfortable, at Dunkin Dounuts. can gateway out thru them all day long. I can NOT however access anything on their local network, it appears to be a simple internet gateway, and that is all (not that I looked TOOO hard).
I personally wouldnt worry too much about informing them, I would just tell them you were using your notebook in the care, and picked up the signal. Hell tell them you were in your contact manager looking up a cellphone number.
Unless you have tried to break into their net, and tripped al sorts of alerts, you really havent done anything wrong. Make sure you document everything the best you can, and I am sure that if they decide to bonehead into causing you trouble, they won't like their name on the front page of the local paper "Bank using unsecure network".
Your call, but I would inform them.

MrCoffee