The Main Idea:
The Phoenix Mars Lander’s weather instrument (a LIDAR – like radar, only with a laser) detected snow falling from clouds above the Phoenix Lander on 9/29/2008. The snow sublimed before it hit the ground, but we’re in Seattle, we’re used to celebrating snow that doesn’t hit the ground.
I know, I’m sorry, I’m really late posting this. But this is a “Mars has Water!!” discovery that I actually think merits some kerfuffle (of the last eight  I think there have been about two of discoveries that do).
So here’s the thing – we’ve known for since forever that there is ice on Mars (though we verified it in 1970), and in 2006 we saw evidence of water that had actually been flowing within the last year. All the other announcements are varying levels of confirmation of these facts. For instance just a month or two ago, Phoenix watched ice subliming. Before that, the various rovers and landers have imaged clouds (that include H2O) in Mars’s atmosphere.
This is DIFFERENT. This is the missing section of the water cycle – not water on the surface, not water evaporating or subliming, not water in clouds, but water falling back down again!
So, if that closed the water cycle what are we still waiting to find? We still want to see liquid water, and I wouldn’t mind seeing some of that falling snow actually hit the ground, but I’m not going to hold my breath for that one.