Update (January 12, 2008): The odds have now dropped to only 1 in 10,000, so the way to bet is on a clean miss by thousands of kilometers.
You may not see too many blogs phrasing it this way, but the odds of nothing happening with Mars have gone done from 99% to 96%.
2007 WD5 is a 50 or so meter wide rock heading toward Mars, and may impact the Red Planet on January 30. However, the odds of a collision are difficult to get. To find out if the two will collide, the orbit of the asteroid must be very well determined, and that's hard. Astronomers have to get a precise location of the asteroid as it moves along its orbit, but that's not really possible. There are errors that crop up from measuring it in the images, from distortion in the detectors, atmospheric distortion, and so on. These add together to make the position of the rock a bit uncertain that far in the future.
The position can be improved by making more observations, finding older ones (thus nailing down a longer arc of its orbit), or by simply waiting until it's closer to D-Day.
It turns out that some older images were found that coincidentally got the asteroid in their field of view, and NASA's Near-Earth Object Program Office was able to refine their trajectory for it: the odds of an impact have now improved slightly from 1-in-75 to 1-in-25. That may sound a lot better, but that's only a 4% impact probability, which means a 96% chance of a miss.
Basically, what this means is that Mars is still somewhere in the fuzzy ellipse of possible asteroid positions on January 30. However, as more observations are made, that ellipse will constrict, and odds are good Mars will be outside of it. But we can't be sure!
I'd very much like to see this thing hit. It won't hurt us here, will only do local damage to Mars, won't hurt the rovers or any other of our robot proxies there, and we'll get awesome information on what happens when an asteroid impacts a planet. This has never been seen before, and it would improve our knowledge profoundly of such events, and that's A Good Thing.