I've checked the site backend, and checked with the monitoring services I use and nothing has popped up as compromised. No other reports as of right now, on site or via Google Alerts, etc. I can't replicate it here with IE and Microsoft Security Essentials either.

While researching, I did find that some folks may have had browser based malware on their computer which switches out ads for ads from shady places, triggering this. I would run a sweep on your personal computer for spyware, malware, viruses, etc using multiple programs to make sure you aren't hit.

I will continue to monitor our end.