With all the talk of Martian meteorites and 'deep thought' about the origin of life on Earth, I thought it perhaps useful to make a few comments myself.
The notion that we can explain the mysteries of life on Earth by saying it originated from spores or whatnot from space is so utterly foolish I'm at a loss for words. That just shifts the question to "Where'd the spores come from?" Yet I see references to this "explanation" in every other news article on exobiology.
On the other hand, why couldn't life from Earth seed other planets in our system? We're learning from extremophiles just how drastic the conditions can be and still find life, and from other work how far down in the rock you can find organisms thriving. So, given that the Earth has had a few impacts over the years, why couldn't some rock chunks land on Mars or Europa with some hardy opportunists aboard?
Unfortunately, it is uphill all the way. Though the Earth's escape velocity is only 11 km/sec, well within even small (100 m) meteorite impacts (Science V298 29-Nov-2002 p1752 Head et al) capabilities, you also have to take into account the potential difference between Mars and the Earth due to the Sun, and that's not small. When you take both potentials into account you need an escape velocity of 26 km/sec to get to Mars, and 40 km/sec to get to Europa. It is probably still doable, but I'd think it very rare.