Unfortunately some American blogger had already beaten me to the idea and recorded himself losing a total of 9lbs just from playing the Wii daily.
While we all wait for a string of fitness games to be released (c'mon it's inevitable, I can see it now: "Jade Goody's Wonderfully Wacist [gettit?] Wii Workout") an inventor has prepared himself for the onslaught of fitness fanatics that are readying themselves to jump on the Wii bandwagon.
Two prototype Wii Weights have been developed of which strap on to your wrists for when playing Wii games, hence strengthening your wrist and arm muscles.
These weights are still only prototypes and the original inventor is awaiting interest from investors. On saying that, I reckon we can expect them in the shops pretty soon.