There is compelling evidence, such as the mark's high infrared brightness in reflected sunlight, that it is black dust resulting from the impact of an asteroid or comet. Jupiter's atmosphere normally contains no dust. Leigh Fletcher twittered from NASA's Infrared Telescope Facility in Hawaii: "This has all the hallmarks of SL-9 in 1994 (15 years to the day!). High altitude particulates, looks nothing like weather phenom." (Keep up with Fletcher's tweets, and read his July 22nd blog post about the impact).
The spot is located near Jupiter's System II longitude 210°. For the predicted times when it will cross the planet's central meridian, add 2 hours and 6 minutes to each of our predicted transit times for Jupiter's Great Red Spot.
If it's really black debris dredged up by an impact, it will probably become spread out horizontally by jet streams in the coming days, and will thin out to invisibility in a matter of weeks and months — as did the marks from Comet S-L 9.
Here's a local article from Australia on Anthony Wesley and his discovery.
Keck Observatory takes infrared images.
Here's a fine two-color mid-infrared (i.e. thermal glow) image from the 8.1-meter Gemini North telescope taken on July 22nd. Note the Shoemaker-Levy 9–like splash ring.
The Hubble Space Telescope team suspended its shakedown and calibration of the recently rebuilt telescope and rushed the new Wide Field Camera 3 into service to image the impact mark. Read all about it, and view more Hubble images.
July 27. More than a week after the impact the mark is still quite visible in small amateur scopes. From Victoria, British Columbia, S&T's Gary Seronik writes:
"I saw the impact clearly with my modified StarBlast reflector [on the night of July 27th]. So yes, it can be seen in a fast 4.5-inch scope. The spot looked like a misplaced, slightly diffuse shadow transit. In the best moments of steady seeing, the impact was quite easy to see." However, he notes, its high latitude on Jupiter means that it spends only a short time in from the planet's limb, "so you really have a window of opportunity that is perhaps as short as 15 minutes. After that, it gets tough — at least for a 4.5-inch scope."
July 29. The situation keeps developing; the mark is elongating into a big, diagonal gash. See Fabio Carvalho's images from this morning.