The latest findings from the Mars Reconnaissance Orbiter offer some promising signs that the early aqueous environment on Mars was not the acidic mess that some had theorized. The presence of carbonates in key strata of Martian bedrock argue convincingly for a neutral-to-alkaline composition of past Martian waters (since an acidic environment would dissolve the carbonate in short order). This in turn suggests that conditions were favorable (or at least not brutally hostile) for the emergence of life as we know it in ancient Martian surface water.
You can see on this map that the orbital assay has revealed carbonate in several equatorial regions of the Martian landscape. The fact that carbonates were also detected by the Phoenix lander on the northern polar plains suggests that relatively hospitable waters were once found in widely distributed regions of Mars in the ancient past.
Does this mean that life once existed on our neighboring planet? Insufficient data. But it should be noted that life appears to have emerged on the young, cooling Earth almost immediately (in geological time). This suggests that the processes which lead to the complexification of organic molecules into self-organizing, pre-biotic hypercycles can occur quite readily when conditions permit. I have a sneaking suspicion that more detailed study will yield evidence of at least primitive life having existed on Mars in the distant past. Whether some life still exists (e.g., in deep aquifers kept liquid by the planet's internal heat) is very much an open question.
As immense a find as that would be, I actually hope we find that life flourished on Mars for one brief, shining moment, then slipped into oblivion. The presence of extant life on Mars would greatly complicate the ethics of eventual colonization, given the immeasurable scientific value of coming to understand the biology of a separate cradle of life in our own solar system. But that's a question for another day. For now, it is very exciting to have begun to solve the riddle of the missing carbonates, and so moved one step closer to the discovery of a "second Creation," and thence to the tantalizing indication that our universe may in fact be teeming with life.
Might make stargazing a rather less lonely experience.