It appears as if some anti-Call of Duty fan is playing tricks and redirecting fans to the Battlefield 3 website instead.
If you go to ModernWarfare3.com (at time of writing), you will immediately be taken into Battlefield 3’s official website instead. As reported by Kotaku, it’s likely a troll that is responsible for this sort of activity.
Neither DICE or Electronic Arts will be responsible for an act like this as Activision will sue them for a lot of money if they did.
According to Kotaku, Activision does not own the domains to ModernWarfare.com, ModernWarfare3.com or ModernWarfare4.com. It looks like someone knew this and altered the domain entirely.
This event has been occurring since last Friday.
Update: The official website for Modern Warfare 3 is http://www.callofduty.com/mw3.Modern Warfare 3 Website Redirects You To Battlefield 3 Website,