I don't think Wii Fit is totally sold out in Europe. Well, the facts don't look like that to me.
Week 1: 661K
Week 2: 337K
Week 3: 251K
That's a normal Wii game drop-off. But that's not important. If it is not selling out, there are a few possible reasons.
1) It didn't sell out first week, and therefore it sold extra second week. Real production is 250K/week.
2) They stockpilled extra and released an extra 100K or so for second week.
3) Stores didn't sell all their copies first week, but sold them second week instead.
4) It didn't sell out 3rd week, and production is really 300K/week.
There are a couple of other things that could be the case too.
5) They are stockpilling for USA launch, and take some from 3rd week, as demand isn't as huge as they expected
6) They have huge pre-orders in America and take some from 3rd week in Europe, as they want to fullfill all pre-orders, and to create some fuzz about the first week sales.
7) Nintendo had so many pre-orders in Europe that first week supply wasn't enough, so they took some of what was supposed to be in Japan over to Europe, and managed to fill all preoders.
8) Same as above, but they also take some preorders from 3rd week and send to America, so that all preoders will be fullfilled.
And another thing, I'm pretty sure you can change the region-coding in the Wii if you know how to (Nintendo does) and it shouldn't be too big a deal. At least I guess so, but this is uncertain (like the Wii)