Arsenic it is... but the point really isn't arsenic. That is what Dr. Felisa Wolfe-Simon stressed at the end of the NASA press conference today - that the research being unveiled is just opening the door to other unexamined possibilities for life. She said "I am interested in exceptions, why aren't things constants in nature?"
Her team's new finding is that certain microbes are capable of creating DNA with arsenic rather than phosphorus as the molecular backbone. These are microbes closely related to well known microbes, but their ability to make this substitution is remarkable, and flies in the face of conventional biological wisdom. This discovery might not be as thrilling as aliens, but it calls into question a major assumption about the nature of life on Earth - that all life requires phosphorus to build DNA and other molecules. There are no other known examples of any other element substituting for any of the 6 major biological elements (carbon, hydrogen, nitrogen, oxygen, sulfur, & phosphorus) in fundamental biomolecules.
the assumption that all life would require phosphorus, and that arsenic is necessarily toxic to life because it resembles phosphorus has been used in predicting zones of potential habitability in our solar system. Astrobiologists will now have to expand their view of what locations are potentially habitable in the solar system and beyond. Additionally, scientists are going to have to start thinking about what other strange biological substitutions they may have been missing.
Until I can give the paper a thorough read and post something more detailed, check out this NYTimes article.
I am still hoping for the discovery, some day, of life on Earth (or elsewhere) totally unrelated to life as we know it, and with research like this continuing, that day might come sooner rather than later.