We may get alot of crap about being Californians but I'd say at least 30-40% of the people that come to my store have reusable bags. There are even signs at the front door of most grocery stores reminding people so they can go back to the car to get theirs! I admit to having scampered back a few times myself but I've found super cute ones that fold up to nothing and I just leave it in my purse.
My store even gives you a five cent credit per bag that you bring in! And aside from all the PC stuff, I can tell you that a grocery bag cost alot more than $.05 so the store benefits as well.
Claudia