Vikings will extend the lease on the Metrodome while waiting for a new stadium to be built... or they could always use the Gophers stadium for a year or two. From what I've read, before they even start considering those sorts of options, they want the stadium being built. They aren't going to do those sorts of things without some gaurantee there's a new shiny stadium at the end of that yellow brick road.
I believe time has almost run out. With two plus years to build a stadium, not proposals, environmental impact studies, road improvements, contracts, and legislature approval, the Vikings playing in a new stadium in Minnesota is timed for the 2013 season.
With the lease up after 2011, I believe Wilf moves the team to a more profitable scenario, or sells the team and the owner moves the team to a more profitable scenario.
Sad for us fans, but the NFL is big business.