Glenn at Instapundit points to a UPI story that says JPL thinks the odds of a 4,000 km flyby for 2007 W(M)D5 or less is 99.7%. If it's not going to hit Mars, that increases the chances that it will slingshot around Mars toward Earth. Odds are likely in the 1 in a million range or less, but what if it did? I wouldn't say "probably", but let's have some transplanetary musings!