Masanao Maeda, corporate director at Sega's game software division, revealed during TGS that he believes the sales of its upcoming Wii and DS title Mario & Sonic at the Olympic Games will go on to sell four million units, a number that would match the success of Sega's original Genesis title Sonic The Hedgehog, which debuted back in 1991.
A timeframe for when the title would reach 4 million copies sold was not given, but hell, I wouldn't be surprised if it reaches that number within mere months. No matter how good or bad the game is, put Mario and Sonic in a game and it's destined to sell millions upon millions.
Sega Sammy Says Sonic and Mario Game Sales May Reach 4 Million [Bloomberg]