On the old forum, someone said there definitely wasn't life on Mars. He argued that life was so pervasive that on any world where it existed, it should be everywhere. Since our probes hadn't found it, it just wasn't there.
Others countered (and I agreed) that just because life was relatively prolifigate on Earth, there was no reason to assume that would be true elsewhere. Mars life might exist in subsurface ice, or caves; it might be a planet of what we would call extremophiles. Or it might be more like Terrestrial viruses, which are arguably not "alive" as we usually define the term (as pointed out in the thread started by One Above All).
Seriously, radiodurans gives me a not-unpleasant chill: if such an organism can exist here, why not elsewhere?