Pat gets a dusty no-prize for bringing us this Nature article detailing recent theories about how the Earth itself may be spreading microscopic life throughout space as it travels around the galaxy. Sometimes I think the "life is how planets reproduce" bunch are full of it. Sometimes, though, it makes sense.
The theory does make some testable predictions, mainly that if we gather up some interstellar dust we should find an earth microbe or two mixed in. The recent stardust probe's collection effort may yield something along those lines. Of course, it just might find non-terrestrial microbial life in the dust out there, which would be beyond cool. We'll find out soon enough.