After learning last week that boiling freezer bags is not recommended, I decided to do a little more research about polyethylene plastic, the main "ingredient" in plastic bags. Low density polyethylene is used not just for freezer bags, but also for vacuum sealing bags (like FoodSaver). So I suspect that whatever type of plastic bags professional chefs are using for their sous vide, they are likely made of polyethylene.
And as it turns out, polyethylene melts. Above 115°C, the polymer changes from a clear solid to a relatively low-viscosity melt. It's hard to figure out the exact melting point for plastic bags because there are many different types of polyethylene (high density, low density, linear low density, etc.) but all seem to melt around the boiling point of water (100°C, 212°F), and some below it.
Even if you think your food will cook well below the melting point, there's still the issue of transference: molecules of the chemicals in the plastic can leach into the food or beverage. These chemicals are added to the plastic during the manufacturing process and some studies have shown they can find their ways into our bodies, especially when heated. So not only is sous vide potentially bad (either the poor man's version or the rich man's), but reheating that lasagna in the Tupperware isn't so great either.
Where does that leave us? Laying off all kinds of sous vide preparations, and migrating home storage containers from plastic to glass, or another inert material. Or just hoping that a little plastic does the body good.