Slate magazine reports on criticism of the recently-released report on bacteria that use arsenic in place of phosphorus. Much of the concern arises from the possibility that the arsenic detected in these biomolecules were not endogenous but instead contaminants. Some scientists are also dissatisfied with how NASA was performing "science by press release", pumping up the publicity machine ahead of the paper's publication.
It remains to be seen how the team that produced this paper will respond; they are willing to give cultures of the strain to any researcher who wants to investigate their claims, and detailed criticisms are presently being prepared for publication.
In all likelihood, though, the mass media will not be as interested in probably prolonged back-and-forth that will ensue, the news cycle isn't quite patient enough for such sustained coverage. This of course is a pity because it biases how the public views science: as a string of spectacular discoveries, but without any of the routine counter-checking that goes on afterward.