Well, I am very interested with this entire freezing / boiling in a vacuum debate.
I just did some research, and I found some very interesting information --
http://www.damninteresting.com/?p=741
This, coupled with some of my readings from other sites, leads to a combination of very interesting things --
I was incorrect, blood would not boil, but the nitrogen in the blood does bubble, and the water within an organism does evaporate.
You
can freeze in a vacuum, but it is not immediate, and depends on the presence of heat energy.
So, all water within a xenomorph would have to be intermingled with another chemical that would drastically increase the boiling point so that it would not react in a vacuum.
Likewise, they would also have to have little or no nitrogen in their blood stream.
However, as far as not possessing lungs or needing to breathe goes, I think that this article just further proves that a xenomorph would have to absorb oxygen or other vital gases in a less direct manner to prevent explosive decompression.
I'll get around to updating this in the biology -- the oxygen from the digestive system theory still stands in my opinion, I just need to fix some stuff ^_^
(Oh, and Kill, I wasn't selectively trying to disprove anything you were saying, I was just responding back to your post)