Updates on Mars and Methane
Astrobiology.net has been running a series on methane on Mars and the possibility that it is being generated by microbial life. I think the bulk of the evidence is in favor of some geological process. The last article in the series goes over how to distinguish living from non-living sources of methane.
And the New Scientist talks about some of the most recent data. Methane is almost certainly being produced continuously, by spot sources.