The impact of a small asteroid or comet into Jupiter's atmosphere, first observed by astronomer Anthony Wesley (let's be honest, anyone who takes the kinds of pictures he does of Jupiter is not an amateur), has been confirmed in observations taken by NASA's Infrared Telescope Facility (IRTF) atop Mauna Kea on the island of Hawaii. The image shown at left reveals the impact to be glowing quite brightly in the near-IR, in a methane absorption band at 1.65 microns
Based on additional images taken by ground-based telescopes, the impactor came in from below Jupiter, striking the South polar region sometime between 07:00 and 14:11 UTC on Jupiter's nightside. Several dark spots in addition to the main impact site are visible with a faint, fan-like plume deposit to the west and north of the impact site. Similar plume deposits were seen at Shoemaker-Levy 9 impacts 15 years ago this week in 1994.
EDIT 07/20/2009 06:23 PM: New Scientist has an article with an image taken by Keck II. The IR data from Keck seems to suggest the possibility of multiple impactors.
EDIT 07/20/2009 11:33 PM: Looks like the image from Keck II in the New Scientist article is a bit of a double exposure, making it look like multiple impact sites.
In case you missed it, an hour ago I posted some of my thoughts on this, the 40th Anniversary of the Apollo 11 landing. Don't forget to post a comment there about when you think the first humans will land on Io (never is a possible answer, but not one I necessarily agree with).
Link: New NASA Images Indicate Object Hits Jupiter [jpl.nasa.gov]