Some breaking news from The Great Beyond blog post linked above:
"NASA experts say the risk of an asteroid smashing into Mars is higher than they previously thought. Last week the agency said the probability of asteroid 2007 WD5 hitting the red planet was 1.3%. After further number crunching it now says it is more like 3.9% (press release). Although this is an unusually high percentage, it is still most likely that WD5 will miss the planet."
The Great Beyond post links to various interviews and articles about this prospect, including a NASA animation.