It turns out that the ad-server-cum-spyware king, Gator, is blocked by my company's firewall as a "Criminal Skills" site.

I discovered this when a popup window popped onto my screen... well, off my screen actually. It appears that the latest tactic isn't a pop-up, or even a pop-under... it's 'pop way the hell outside of your resolution range'. When I moved the window into the viewable area of my display, I was able to see the error message.