I don't mean to recycle a tired, old topic here, but I was recently asked by a newspaper columnist for an opinion on the relative impacts of paper, plastic and cloth shopping bags. This is interesting once again because of the plastic bag ban in San Francisco, to be followed this year by my hometown Portland. The ban is now common in parts of Europe and Asia, including Bombay and Bangladesh. The litter and the difficulty in recycling these bags are real problems, so I am personally glad to get rid of this nuisance as long as there is a viable replacement.
We did a carbon footprint study of paper and (somewhat heavy-duty) plastic for a grocery chain a couple of years ago, which showed that plastic had the lower carbon footprint. In response to the recent question, I dug up an Australian study that provides comparable material weights for HDPE plastic, Kraft paper, reusable non-woven polypropylene (plastic "green bag"), and reusable light-weight cotton/calico grocery bags (all scaled for equal capacity, about 7 grocery items per bag).
A quick carbon footprint analysis shows that the reusable plastic "green bag" is a good option: it takes about 18 uses to break even with single-use HDPE bags and just about 4 uses to break even with single-use paper bags, and additional uses after that are basically emissions-free. On the other hand, the light-weight cotton bag takes over a 100 uses before it breaks even with HDPE. This assumes that the cotton bag is washed rarely, the paper bag is recycled after use, and all plastic/cotton bags are landfilled at end of life.
The "green bag" would easily win a life-cycle carbon footprint contest.
I have looked at just the GHG emissions and this is only a quick ball park analysis. But I do hope that the plastic bag bans are accompanied by decent information on what else to use.