To answer the question YES it is possible to get a virus from the ads...
It happened to me last week on a machine at my office by going to YAHOO.COM.
In checking things out one of the ads on YAHOO had a payload in one of the FLASH ads being displayed. The computer that got hit had anti virus software on it. By the time the software caught it it was too late and the computer already was infected by something that looked like a virus scanner.
Took some work to get rid of that one.
I was told that if I was running the latest version of Flash that it would have never happened.
Here at SatelliteGuys we get our ads from 3 networks... Burst Media, ADSDAQ and Google.
From what I have been able to track down from the clues people have given me, it appears that on the weekends ads being served by one of the networks displays ads that have been hijacked. I have been in contact with that ad network and they have admit that some of the 3rd party ads they have been re-feeding to us might have had issues and they were working to insure that it would not happen again. Since then (and it has been awhile) there has been no report of issues. (Until this thread)
If you see something like this please PM me with all the details possible, including description of the ads being shown, where the ad was on the screen, and also if possible a screenshot of the ad. This will make it much easier for me to track down the ad, and also the agency.
If you haven't already make sure your version of flash is up to day as that is how that stuff is getting in. Having an old version of Flash is like keeping your doors wide open when your not home. Trust me, I learned this the hard way myself.
