A DNS Changer perhaps? A just-too-be-sure scan with iAntivirus wouldn't hurt.
I bet this is what it is. There is a list of certain DNS numbers you can check in sys prefs/ network to see if you've been infected. I cant find the list of them now. It happened last year sometime.
Most often it comes from porn sites, but doesnt mean it was from a porn site. It prompts for the user to install a required video codec to view something and the trojan is installed. If you google around you can find the details of it.
From what I've read you have to use a DNS removal tool, anti virus of some sort, or actually reinstall your os.
http://www.dnschanger.com/