I found this online - looks like it's 330lbs.
The Wii Fit is AWESOME!!!
Here is an excerpt from an interview with Miyamoto himself as he explains why, and of course I'll put the link in the source.
"The one other point that I probably should explain is that, in Japan, you might notice on the Wii Fit box that the Balance Board is rated to 136 kilograms, or 300 pounds, whereas here in North America if you look at the Wii Fit box it is rated to 330 pounds, which is 150 kilograms. So, looking at the boxes, you would think that the Balance Board was made stronger for here in the United States. In fact, Japan has a law called the law of weights in measures, where scale manufacturers have to submit their products for certification from the government, and the standards that they were using at the time, 300 pounds was the standard for a household scale. So in order for the Balance Board to receive certification from the Japanese government under this weights and measures law, we had to certify it at a level of 300 pounds. Because the United States does not have that law, were able to state that it functions at 330 pounds, despite the fact that the boards are exactly the same in the two countries.
Also, because when youre standing on the Balance Board for some of the activities you dont quite jump but you squat and exert different amounts of pressure on the Balance Board, the actual physical structure of the board is able to withstand much greater weights of around probably 660 pounds. The weight that it can accurately measure is 330 pounds"
-nat
We got one for the first night of Hanukkah and have been on it every day since. For those that dislike exercise, the Wii Fit rocks; you don't even know you are exercising! 
<--------------Me hoola hooping. 
the one we got! We've had for 2 days and we've been on it constantly and you really can get a workout - I absolutely addicted to the Aerobics portion of it. My DDs are trying to beat their best scores as I'm typing this 
My wii fit age is 21 and I'm 29 but it also says I'm obese and my mii plumped up. That was funny. I can't quite get the hang of the step aerobics...those darn mee's waving their arms keep messing me up. 
It says I'm overweight. But my family says that I need to start eating more! So who knows! 
You should try it... it's really funny when it says your WAY under weight!
Love the yoga.
