Our official position is that our Ziploc® bags are not recommended for cooking, only for reheating and defrosting.But let's be realistic. Heat is heat whether you are "reheating" or "cooking". With further prodding I was able to find out that Ziploc® bags start to melt at 230F and no, they don't contain BPA (the chemical that made us all throw out our water and baby bottles). So, what does all this mean? Are they officially safe? No, because they were never tested at low temperatures for long periods of time like the vacuum seal bags -- those are officially safe for sous-vide cooking. However, I don't see any big red flags here telling me bad things might happen if you occasionally use Ziploc® bags to cook something sous-vide. That's my personal conclusion and you need to use your own judgement when making this decision.
This closer look at cooking in plastic made me take a loser look at cooking WITH plastic too. When I looked at my plastic spatula, I didn't like what I saw. It was easy to see with a naked eye that it was melted. After inspecting it carefully, I found tiny little letters on the back of its handle (black on black) that read "Safe up to 400 degrees." That's like making a car that is safe to drive up to 30 miles an hour. What if you ever want to get on a highway? Any time you are cooking in a skillet, you are likely to get way above 400F. Saying that you shouldn't use it on "high" heat is way too vague. Even on the same stove, medium of one burner is not the same as medium of another burner. A lot also depends on how long the pan was sitting on the burner, what was in the pan and how the food was positioned.
What makes us question Ziplock bags more than the spatula? I am guessing it's the novelty of it. Very few households cook anything sous-vide, yet I can't think of any household that don't have a spatula. It's just like driving vs. flying. I haven't met many people afraid of driving, but many that are afraid of flying. Statistically, you are way more likely to die while driving than while flying, but the fact that you drive every day takes all the fear out of it.
The plastic spatula is now safely residing in my trash. I replaced it with metal. Technically, you shouldn't use it on non-stick pans, but I'll take my chances with that. I found that if I am gentle and don't scrape the pan with a spatula, nothing terrible happens.