Could Earth collide with Mars?

[Tony G]

Scientists believe the Earth could one day collide with Mars or Venus, because of the unpredictable orbits of planets. 

Experts say there is only a tiny chance of it happening and it would be unlikely to happen for billions of years.

http://news.bbc.co.uk/1/hi/uk/8094088.stm

Tony G

[Rick]

Forget sending astronauts to Mars, the planet may come crashing right here to Earth if gravitational interactions substantially agitate its now-stable orbit.

A new study in the June 11 issue of Nature predicts there's a real, albeit slim possibility of a planetary smash-up inside the inner solar system, largely thanks to Mercury's distinctly lopsided orbit.

More: http://www.theregister.co.uk/2009/06/11/boffins_track_odds_of_planetary_smashup/