I don't see why Mario Kart's online mode wouldn't free, considering that both Strikers Charged and Brawl have free online. They're all big Nintendo franchises with similar online play requirements, why create a dividing line there?
I'm with those who say the pay-per-play label will be for specially selected titles such as MMOs.
My Mario Kart Wii friend code: 2707-1866-0957