This is a great issue, and definitely worth some design discussion. But an important thing to remember is that the previously mentioned “Julie” profile is most likely already eco-conscious, and definitely not the largest contributor to the problem. In fact, she may already participate in voluntary bring-back programs for PGBs.
Instead, Allow me to present Marie and her family:
While they aren’t really opposed to safeguarding the environment, with 5 people’s worth of groceries (two full carts!) to lug home solo every week, Marie is hardly going to consider anything which might add hassle to the prospect.
However, Marie is very sensitive to style, and keeping up with what the new “suburban mom item” is, particularly those items she sees on shows like Oprah, and Desperate Housewives (Oh come on, you know you watch it 
What if we could make something cool enough to replace bags that it would make “Oprah’s Favorite Things”?
What if designers stopped trying to “Explain first, convert later” and used the power of mad consumption to our advantage? What if we weren’t making a bag that was “Just reusable” (And probably would get misplaced before it could really make an impact – if you throw away a sturdy woven plastic bag, or even a cotton one, that’s a lot of energy you wasted compared to a plastic cheapie)
What if we were making a new system for getting groceries home
For example:
RFID all groceries (OK, this is big, but stick with me). Now, just pile all groceries into the cart – which is a tub now, not a mesh basket. Now just walk out the door with everything – it’s paid for. Now, you walk to your car, and dock the tub in the back. An automatic mechanical system slides an empty tub out of the car, and the full tub inside. The empty is then replaced in the cart, and the cart returned.
This is VERY blue sky, and will probably never happen, but remember that the big problems and big solutions come from big changes. Paper bags were already a waste generator before plastic – the real waste problem came when society changed it’s mind and saw bags as something not valuable enough to re-use.
So now, we need an equally huge idea – like that change of mind – to make bags completely irrelevant. No bags, no waste. There’s no reason to think small in a contest like this.