I seriously doubt Wii will ever reach 50 % of total sales for this generation. I'd say 48 % will be it's highest overall marketshare, and it will reach that level sometime in early 2009. Beyond 2009 the PS3 will slowly eat into Nintendos market share. However, I don't believe PS3 will ever catch up on Wii.
My prediction as of March 31 2009 is like this:
Wii 45,0 - 48,5 million
PS3 25,0 - 27,5 million
Xbox 360 24,5 - 27,5 million