Yeah, it IS strange that Nalgene FAQ's have no mention of it.
I found the page while googling HDPE vs. Lexan bottles and it came up. HDPE is the plastic found in military 1 quart canteens and the like. Nalgene still makes HDPE bottles, they're sortof a snowy white.
There's really only two problems with HDPE:
1) It absorbs the smell from whatever you put in them. That's why other than water, it's not such a great idea to put anything else in them. (But you can clean them out with baking soda or lemon juice)
2) It's heat sensitive. If you put really hot water into an HDPE bottle or canteen, it goes mushy and soft. It won't melt, but it becomes really pliable.
Personally, I prefer HDPE because it's just more durable, I find. Sure, I don't put anything other than water into my army canteen, but that's what the canteen cup and kidney stove is for. Lexan looks nice and you can put anything you want in it including hot tea, but carcenogenics? I'm a little wary now.
Adam