Accoring to Games Industry, Nintendo UK general manager, David Yarnton said that the company has seen 3 million connections take to Nintendo's online WiFi service. That means there are roughly about half-a-million actual users.

This awesome success can be attributed to the successes of Mario Kart and Animal Crossing: Wild World.
The best thing about this is that Nintendo can now look forward with very high optimism levels for its immediate future as the Wi-Fi enabled Metro Prime Hunters is about to be released soon for Nintendo's handheld device.
Via: Games Industry