The Kepler Mission is a spacecraft that stares at the same 100,000 stars. This is all for the sake of discovering Earth-sized extrasolar planets by detecting variations in the brightness of a star with periods that can last over a year. It would be impossible to allocate years of telescope time for observing just a single star, and even if an astronomer manages to get that time, the star might not even have a planet, which is why Kepler observes 100,000 stars all at once. Not to mention that thousands of planets could be discovered this way at once, instead of praying that the single star you are observing has a planet.
Given Kepler's goals, it would be nice to make sure that it could actually detect planets. And as reported at Bad Astronomy, Kepler has managed to detect the known planet HAT-P-7b, an extrasolar planet that orbits its parent star with a period of 2.2 days. Now that Kepler has been shown to be able to detect known planets, we can be much more confident that it could indeed discover new planets, hopefully including those that are as small as our own Earth, without having to twiddle our thumbs for years wondering if Kepler would even work.