OK, I'll admit I'm a bit late to the party on this one, but I just saw this "accessory" for the Wii called the Riiflex. It gives new meaning to Wii Fit. The company that makes these is Power Play and they offer a 2-pound and 4-pound pair of dumbbells. The company is taking preorders for them and the weights should start shipping this summer.
There have been some pretty lame accessories out there from the Wii remote, but this actually seems fairly useful. Granted it's not going to buff you like Arnold, but it could tone the arms up a bit, especially if you tend to flail wildly as I do in some games. The only drawback I see is that I expect my right arm to get more of a workout than my left, since I don't use the nunchuck portion nearly as much as the remote itself. That would sure make for a lopsided look. Guess I will need to learn to do some Wii gaming left-handed.