Er, Mars has always had an atmosphere, just not terribly thick. We're still not sure how dense and of what composition the earlier atmosphere was, but judging from the plentiful signs of massive amounts of liquid water that must have once flowed on the surface, it's a fair bet that the old atmosphere was capable of supporting life. Getting life transported from Mars to Earth, though, is a really big stretch. We've found martian meteorites, but those are one in a million, and the odds of having genetic material withstand being excavated from Mars, traverse the inner solar system, be subjected to all that radiation, and finally impact on Earth, is beyond miraculous.
IIRC, the more central purpose of the lander was to determine if there was sufficient water ice in the polar regions to be a viable water source for future human inhabitants. We already knew there was CO2 ice at the poles, and we've imaged water-ice in the form of frost from previous landings, but here we can actually see what appears to be significant amounts of water ice just under the polar soil. (It's actually quite similar to subsurface permafrost on Earth).
If they find signs of life I suspect it's most likely little more than simple microbes. It'd be awesome as tits if they do.
And I actually highly doubt that they would hide this information from the public if they found anything definitive. What would anyone gain by keeping such knowledge from us? Besides, only extremist religious folk would be likely to raise a hissy-fit over it.
