From the Ziploc.com FAQ: "SC Johnson does not use BPA in its plastic products, Ziploc® Brand bags and containers, and Saran™ brand wraps."
On the other hand they say don't use the bags with boiling water, but do condone use in microwaves where steam and fats can reach much higher temps.
In our family we have completely eliminated using plastic for heating or hot food at home. Camping is another story, but since at the most we spend less than 15% of the time camping I think the convenience is worth the risk. (Most of our family camping is car camping and we don't boil in the bag there anyway).