The point of this mode of cooking is that it's low temp, below the temperatures at which plastics in high-quality freezer bags become an issue (Ziploc confirmed this to Dave Arnold earlier this year).
Make sure you're using Ziploc specifically -- as in that brand. Other manufacturers may have their own formulations for their plastics, including non-heat-stable plasticizers. I contacted Trader Joes, for example, when writing Cooking for Geeks, and they disclosed that their plastics are NOT heat safe.
