How common are Earth-like planets?

NASA's Kepler Space Telescope is nearly ready to launch, with liftoff scheduled for 5 March.

The mission will attempt to identify potentially habitable exoplanets (that is, those having a rocky crust and that lie within the "Goldilocks" zone around other stars) using the transit method. As these small planetary bodies pass in front of the star around which they orbit there should be an observable dimming of star light, and Kepler's optics are optimized to detect such minor decreases in stellar magnitude.

Faster, please.