When Valve decided to elevate Portal from a curious oddity on the same disc as Half-Life: Episode 2 to something people would be paying full price for, there was a risk it'd backfire. At the Games For Change conference in New York today, Valve co-founder Gabe Newell showed Portal 2 was doing just fine, announcing Portal 2 has sold three million copies so far, Joystiq reports.

Portal 2 sold 637,000 copies across Xbox 360, PlayStation 3 and PC at retail in April, according to the NPD Group. The sales for May were not released, but what NPD Group doesn't account for are sales through Steam. While Newell didn't explicitly say the three million count includes Steam sales, and I've contacted Valve to verify just to be certain, one could reasonably assume they do.

The other caveat is that Newell didn't say the number encompassed all platforms, but, again, it's reasonable to believe so.

To date, the original Portal has sold around four million. That doesn't include Steam, either!

If you've already exhausted Portal 2's single-player and co-op content, the first of possibly several downloadable add-ons is coming this summer. The best part? It'll be free everywhere.