More info on slashdot:
Slashdot | Hackers Use Banner Ads on Major Sites to Hijack Your PC
And a video on explaining the thing and why it's such a bitch to track:
YouTube - Major League Baseball Strikes out
I've been seeing this ad for quite a while now (months), very rarely though, only once a month maybe, but due to the redirecting nature it's really difficult to see where it comes from