Sometime in the next two weeks, the Wii will eclipse the XBox. :)

[Nickelbackro]

Sony PlayStation 2 120 million
Sony PlayStation 102.49 million
NES 61.9 million
Snes 49.1 million
Nintendo 64 32.9 million
Atari 2600 30 million
Sega Mega Drive/Genesis 29 million
Microsoft Xbox 24 million
Nintendo Wii 23.61 million
Nintendo GameCube 21.72 million
Microsoft Xbox 360 18.13 million
Sega Master System 13 million
Sony PlayStation 3 11.60 million
Sega Dreamcast 10.6 million
NEC TurboGrafx-16 10 million
Sega Saturn 9.5 million
Sega Mega-CD 6 million
Panasonic 3DO 2 million
Source Wikipedia and VGChartz

I'll be excited when Wii passes Atari 2600, that's always been the benchmark for me on a successful console, although Sega Genesis is close enough, so I'll give them the nod too.