Load up FireFox and NoScript and then look at the number of scripts running on your site.
You'll find your page(s) load no more than 12 scripts of which I'd rule out doubleclick, googlesyndication, google-analytics and probably msn.
That leaves 8 other ad networks, some 3rd party networks change each time the page loads, and any one of those could be the source.
What's going on is nested ad networks and somewhere along the way it would appear that one of the ad networks is loading ads from an infected 3rd party ad server.
However, today I didn't see anything suspect, maybe I didn't view enough pages to find it, maybe it was already caught?
Hard to say.
However I've seen this garbage before and it's why you should avoid the lower tier ad networks that allow 3rd party ad serving.