We'll let Harold McGee answer this one. On August 11, 2008, he answered this very question in the New York Times Diner's Journal.
Q: Are conventional zip-loc bags safe for sous-vide cooking? If so, up to what temperature? There seems to be a lot of guess work and misinformation (?) about this one on the internet. Love your books! Thank you.
— Posted by Ben Fambrough
Harold McGee replies: Heavy-duty Ziplock bags are made from polyethylene and are approved for contact with hot foods. True sous-vide cooking involves vacuum-packing the food, which zipping a bag won’t do for you. But you can certainly use the bag to immerse food in a water bath whose temperature you control carefully. It can be hard to squeeze out all the air, so the bags tend to float and heat unevenly unless you weigh them down. Sous-vide cooking generally involves water temperatures between 120 and 180 degrees, which the heavy-duty bags can take.