The funny thing you guys as missing here is that practically ALL residents of San Francisco are in favor of this. It encourages people to bring their own bags, and I'd rather have the city generate some revenue this way than raising my property tax. I can't stand those damn plastic bags, and I have always brought my own bag to the store... the problem is that other, less considerate people, let them loose in the wind, and let them overload the landfill which MY TAXES pay for. Furthermore, it is VERY unlikely that this tax will happen in suburban areas (where I suspect most of you live) because the public wouldn't stand for it - in SF, the majority of shopping trips are walking trips to the store to get only one or two bags full of stuff. On the rare occasions when we get in a car to go on a major shopping trip, then we'll suck it up and pay a little tax, not that big a deal really.
As for the poor - it's very easy to get your hands on free canvas bags, and some stores plan to give them away - it's great PR.