This summer does seem to be Wii's prime opportunity to get back some marketshare. I don't think it will push it to 50%, but it could very well push it back to 49%
It'll be back to gains in marketshare, rather than the stagnation/losses we've seen as of late. Monster Hunter 3 and Wii Sports Resort should give the console some great strength in Japan, where the marketshare losses have been greatest, and Wii Sports Resort as well as the other sports games should help elsewhere
Plus the fact that neither PS3 nor 360 are getting much in the way of major titles until fall, except for inFamous and Prototype, which shouldn't be that great of hardware movers, so the advantage this summer, worldwide, goes to Wii.