The geekosphere has been buzzing over yesterday's announcement from NASA about the discovery of arsenic-based life. Specifically, a species of single-celled archaean living in hyper-salty, hyper-alkaline, arsenic-laced Mono Lake in California can actually use arsenic instead of phosphorus in its biological processes. If you Google it, there are already thousands of articles talking about how this dramatically expands the possibilities for life beyond Earth. In other words, we have an example of Life Not As We Know It.
Well . . . no.
Here's why: the Halomonadaceae didn't evolve out of prebiotic molecules in the hostile environment of Mono Lake. They're descended from much more ordinary archaeans, which presumanbly had normal arsenic-free DNA and other chemical machinery. The Halomonadaceae evolved the ability to live and thrive in the horrible water of Mono Lake, but they are descended from life forms which started out in a much more benign environment.
This is important because it means life may be able to survive in all kinds of strange and extreme environments, but it says almost nothing about what kind of conditions you need to start life in the first place. By analogy: one could find humans on the Moon in 1969. That doesn't mean humans evolved there. So if at some point we find a planet covered with Mono Lake water, I wouldn't expect to find alien organisms resembling GFAJ-1 there -- unless somewhere, at some point in the planet's history there were lakes or seas without so much poison. Life is tenacious and adaptable, but it's hard to create.
On the other hand, the fact that GFAJ-1 can apparently build DNA using arsenic rather than phosphorus means there is likely a wider range of possible genetic molecules than biologists had expected. There may well be organisms elsewhere in the Galaxy which use very different molecules as the "information storage medium" for their genes.
Fortunately for science fiction fans, writers and designers have already done work on what an intelligent halophilic organism would look like.