Speculation about the possibility of earth-like microbial life on Mars took a little bit of a hit last week with publication of the thermal history of a piece of Mars. A rare class of meteorite, known as nahklites, are thought to actually be pieces of Mars which fell to Earth. The evidence in the article in Science suggests that these rocks have been very cold for almost their entire history on Mars. This would imply that the planet as a whole was never warm enough to support widespread near-surface liquid water, which in turn means that microbial life may never have had a chance.
In the Science study, the scientists re-analyzed old measurements of the ratio of radioactive potassium to its decay product argon. These analyses (and other radio-dating pairs) indicate that the meteorite ALH84001 (called AL by his friends) was formed underground on mars about 4 billion years ago. AL was kicked out of Mars (about 15 million years ago) by a not-very big impact, so he must not have been buried more than a few kilometers deep.
Argon formed by the decay of potassium will diffuse, and the warmer the rock gets, the faster argon will leak away. The scientists extensively modelled how much argon AL has still in him, and conclude that he could never have been very warm-- otherwise he'd have a whole lot less argon than is observed. The most likely temperature range they determine is around -60 degrees celsius, far below the freezing point of water. They conclude that the kilometer or so nearest the Martian surface has been in deep freeze for its entire history.
It's a little hard to see where this fits in with other Mars data. The Mars Rovers, especially Spirit, have observed lots and lots of rock formations which must come from exposure to liquid water. These are not just one-off volcanic rocks, but sedimentary layers requiring thousands of years to form. Of course, the Rovers' landing sites were preselected for the maximum chance of seeing just such formation; whereas AL and his nahklite relatives come to us from (presumably) a random event.
In the next few years the Mars Express MARSIS experiment will be using ground-penetrating radar to look for current subsurface water.