That's a really hard question, I think the Wii will sell better then PS3 and Xbox 360 during the holidays thinking of all the games coming out: Metroid, Galaxy and Brawl etc.
So my guess is: March 2008
Ah, Damn! Are you talking abou combined WW monthly sales? PS3 + Xbox 360?
Predictions for December 31st 2008:
Wii 38,000,000
DS 84,500,000
PS3 17,000,000
PSP 41,000,000
X360 23,000,000