WD5 most likely missed Mars, but we may never know
I just chatted with Steven Chesley of JPL's Near Earth Object program about the fate of 2007 WD5, the could-have-hit-Mars-but-most-likely-didn't asteroid. He said that based on the orbit predicted from the last observations of the asteroid, which were taken on January 9, they think it missed Mars by 6.5 Mars radii, which is a near miss in solar system terms but very far from the bull's eye if what you really wanted to see from the encounter was a new crater on Mars. (For those of you interested in the statistics, I asked him what the 1-sigma and 3-sigma uncertainties in the miss distance were; they were 1.8 and 2.2 Mars radii, respectively.)
Now, a 6.5-Mars-radius flyby is quite close enough to alter the course of WD5 significantly. Where's it going to go next? The answer is, no one knows. The uncertainty surrounding WD5's exact position with respect to Mars during the encounter hugely balloons when you try to figure out its future path. It essentially got a gravity assist from Mars, but without knowing exactly at what distance and, equally importantly, at what latitude it flew past the planet, we have no idea where Mars flung it. Mars isn't big enough to send an asteroid right out of the solar system (only Jupiter has strong enough gravity to pull off that trick), but a close flyby can really seriously change the orbit of a small body like WD5.
So, Steve told me, WD5 is now considered lost. That may sound sad or possibly even dangerous, but Steve emphasized to me that becoming lost is quite the norm for objects as small as WD5, which is estimated to be roughly 50 meters in diameter. Fifty meters is big to a human, but it's actually too small to do a whole lot of damage; it'd produce a roughly Tunguska sized air blast if we were unlucky enough to be in its future path. There are kajillions of 50-meter objects in the solar system, and we usually only spot them when they are really close to us, track them for a few days or weeks, and then they are lost to our telescopes. WD5 was tracked much longer than most comparable objects because there was so much interest in its potential Mars impact.
It is within the realm of possibility that WD5 could become un-lost. If its new orbit happens to carry it into Earth's neighborhood again, a sharp-sighted telescope -- most likely an automated one belonging to one of the new, second generation near-Earth asteroid surveys that will be coming online soon -- may spot this faint body and send observations of its position to the Minor Planet Center, which will give it a new designation. If we are able to track it long enough, we may develop an orbit solution that shows this "newly discovered" asteroid to have been awfully close to Mars on January 31, 2008. If all that happens, we may link the new discovery to 2007 WD5. However, as with most other bodies that make close approaches to Earth, it's actually fairly unlikely for it to do the trick again, at least in the next few hundred years.
Thinking about this close approach made me wonder if Mars experiences more or less of these than Earth. I can think of arguments in both directions: Earth has higher gravity than Mars and is closer to the Sun, so should be in a much more energetic shooting gallery than Mars; however, Mars is close to the asteroid belt, so should have more bodies in its neighborhood. Steve told me there haven't been detailed studies on this but that he tends to think that these competing effects cancel, so that Mars and Earth have roughly comparable impact rates.