Around 1330 UT (0830 Eastern) yesterday (19 July 2009), an Australian amateur astronomer named Anthony Wesley noticed a previously unseen dark spot on Jupiter. Further observations by Mr. Wesley and other observers show several more spots in the vicinity, indicating that this is likely the result of a sizable asteroid or cometary impact on Jupiter.
If this was indeed a comet or asteroid impact, it would be much like Comet Shoemaker-Levy 9, fragments of which which hit Jupiter in July 1994.
The Spaceguard project is still cataloguing large objects in orbits which might be in a position to hit Earth. However, no one noticed the Jupiter impactor prior to Sunday. Jupiter's larger size and greater gravity causes it to sweep up a lot of material, which is a good thing for us -- every large object swept up by Jupiter is one less to fall out of the sky without warning on us.