On your marks and get set for the world’s greatest sporting event as Mario & Sonic head to the London 2012 Olympic Games. Coming exclusively to Wii and for the first time on Nintendo 3DS, Mario & Sonic return in an all new celebration of the Olympic spirit. Hosted within the truly iconic setting of London, the games are back, bigger and better than ever before, bringing even more multiplayer fun and excitement to friends and families everywhere. FEATURES Exciting headline events - Wii All new events including Football, Showjumping and Canoeing will debut alongside redefined favourites, such as 100m Sprint, Aquatics, Table Tennis and more. Combine these with a new roster of the much loved Dream Events and the players now have more choice than ever before.
Officially Released: Wii..
U.S 15 Nov 2011
Aus 17 Nov 2011
Eur 18 Nov 2011
Jap 8 Dec 2011

Officially Released: 3DS..
Aus Feb 9 2012
Eur Feb10 2012
U.S Feb 14 2012
Jap Spring 2012

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Despite the official release date for Australia, I can confirm the Wii game is available for purchase at retailers as I type.

Source: www.mariowiki.com