There's no way the Wii is not the top selling console by the end of the year, unless they can't produce as many units as the X360.
If Nintendo is really facing production problems, then it may be a tie. Because in that case, they can't build more consoles then they're doing now, so they can't sell any more either.
Unless, of course, they've solved those problems, or managed to slowly build up a little supply for the holiday season.
I expect Microsoft to increase their last two months' X360 sales about 50% compared to last year's.
Sony is a big question though, we need to see sales data for august and even september, just to make guesses.