Fromn what I've seen in teh screenshots and read in various web sites, here's my theory:
Bowser has traveled to the land of Sunshine Town (I made up that name) and has captured the Sun, the main power source that makes the citizens of Sunshine Town happy and free from evil. By capturing the Sun, Bowser plans to use it's power to create an enourmous "evil-ray" that will bring death and darkness to the world...
Mario hears of these plans and immediately travels to Sunshine Town, where the inhabitants tell him what Bowser's evil intentions are. They also explain to him about the Sunshine Coins. Sunshine Coins are mini portions of the Sun that contain small amounts of energy. They say that using the Sunshine Coins will create a small amount of light and will help to ward away the evil, but they only last for a limited time. So the citizens tell Mario that in order to bring complete happiness throughout the world of Sunshine Town, he must collect all of the Sunshine Coins and combine there power with water to create rainbow pools of happieness throughout the town. That's what that backpack on Mario's back is probably used for: to collect the Sunshine Coins and combine their power with water and create small rainbow pools of happiness!
That's just a theory of what the game will be like... Pretty good one, huh?
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MICROSFT: Nintendo is so old, you should retire! You will fall to the wrath of Microsoft!
NINTENDO: Well, at least WE know how to make good games! Long live Nintendo!
SONY: I like cheese...