So says Sarah at Freezer Bag Cooking, who notes the many safety-related questions she receives about people wondering if cooking food in a Ziploc bag will give them cancer. Also:

Freezer bags are made of 100% polyethylene. Most bags are 3 mil thick. And yes, another often asked question: freezer bags are recyclable! Very much so. When you get home, you can rinse out the used bags, and toss in with your plastics.

Her opinion is the bags are safe when using her cooking method: pouring hot water into a bag with dried food in it (which is much different than, say, trying the boil-in-bag thing, which weakens the bag and exposes it to the risk of melting if it touches the pan).