For all of you who wondered about the results of my experiment with the Sun Chips compostable bag, I can now report my findings.
After 8 months and a very hot summer, the bags are still intact. There is compost above and below the bags, and the original bag that started the whole experiment back in March is looking remarkably fresh at the bottom of the compost bin. Even the cut up bits of bag haven't decayed at all.
I must confess I'm a lazy, "compost happens" kind of gardener. I never turned the pile, I didn't layer it properly, I didn't water it during the long dry spells. Therefore, the only conclusion that can be drawn here is that the bags do not break down as readily as organic matter and are best left out of the compost if you have a cold pile and really don't give a hoot about turning, layering or watering your compost.