I decided to try to use my Nalgene bottle as a pressure cooker by sealing it and putting it in a microwave. The result was fairly positive, the water was clearly at higher pressure and got to a higher temperature as a result (PV=nRT). However, I don't know how to measure the pressure and I haven't yet tried measuring the peak temperature.
My two experiments: First, I put a nalgene in a small microwave on its side. It was good for 5 minutes, so I put it in for another 2, and after that it "exploded". The bottle didn't break, but it started spilling. I believe the lid stretched enough to relieve some pressure.
Second, I put a quarter-full nalgene in a taller microwave, standing upright, and put it in for 7 minutes. I could hear steam leaving the bottle through the space between the lid and the bottle, but very little escaped. After I removed it from the microwave, I left it sealed and the water inside continued to boil for about 10 minutes.
So, why would you want to use a nalgene as a pressure cooker? If you want to make espresso but don't have an espresso machine, it's a good option (espresso is made by running high pressure hot water through coffee grounds). I think it will also work well for making other concentrated liquids, e.g. concentrated teas, for mixing in to other drinks.
Health concerns: the water gets much, much hotter than normal boiling water, don't burn yourself. Also, some websites have expressed concern about the plastics leaking into the drink when a nalgene is microwaved.
Other alternate uses for nalgenes?