The Mountain Dew Game Fuel site is officially live, and we're all free to earn lots and lots of tokens. You'll need to register first, so be prepared to hand over a fair amount of personal information. They want the usual things, like your name and birthdate and home address. If I had to guess, I would say they need all of that for contest reasons. You always need to ask those things so you don't run into any regional legal issues.
Now, tokens. How do you get tokens? There's a full list of ways to earn them on the official site (that you need to be logged in to see,) but I can tell you this: Almost all of them involve clicking on some random links scattered around the internet. Watching videos on their site, clicking banners on Facebook, going to their Twitter page, all of that sort of thing. Time to start setting up a Mountain Dew bookmarks folder, folks.
There's also an FAQ that confirms one of our suspicions: Yes, a WoW vanity pet is one of the potential prizes you can win. It doesn't state what it is, but we can all safely assume it's the battlebots we posted about previously. Among the potential prizes is a trip to BlizzCon as well, so if you can't afford the trip yourself, you ought to start hoarding tokens.