Nature has a short opinion piece about NASA's astrobiology initiative [1].
A common misconception is that astrobiology is equivalent to the search for life elsewhere. Some people have even gone so far as to say that it is a science without a subject because we don't yet have any evidence for extraterrestrial life. But that is a flawed argument that has been put to rest on several occasions (see, for example, ref. 10). Many experiments in science target hypothetical particles or objects; biology is simply handicapped by the fact that first principles and mathematics provide limited predictive power. In fact, the search for life beyond Earth is just one subset of astrobiology. As Knoll et al. have written: “astrobiology can be thought of as the application of geobiological principles to the study of planets and moons beyond the Earth.”
The essay points out that NASA changed its biological science organization in part to address the failures of confidence resulting from past efforts (e.g., Martian meteorite fossils). The authors also discount the search for "alien" environments on Earth, after a short reference to the recent "arsenic-eating bacteria" story.
And some of the attempts by other researchers to extrapolate to other parts of the Universe the ability of microbes to adapt to extreme environments may be due more to the struggle for funding than to the desire to study habitable planets.
As long as our sensing systems are adequate only for observing the chemistry of bodies in our own solar system, extreme environments and exotic chemistry are the only game in town for astrobiology. But a new generation of space telescopes may able to collect evidence about biospheres on planets outside our solar systems, which means that "ordinary" terrestrial ecosystem science may have renewed relevance. There's a chance that organic chemistry and atmospheres rich in free oxygen will spring up all over the place. If so, astrobiology will look an awful lot like terrestrial biology. Meanwhile, recognizing signs of exotic life chemistry on distant worlds may be harder.