Speaking at Games For Change in New York City (right now), Valve's Gabe Newell casually noted that Portal 2 has sold 3 million copies since its release on April 18. He didn't specify platforms, as it was an incidental reference and not a press release -- but he's probably referring to all platforms (PC, Mac, Xbox 360, and PS3).

Since Newell is one of the only people on Earth with access to Steam sales data, that 3 million number is likely to be one of the most accurate estimates of Portal 2's sales we'll ever see. For comparison, the first Portal had sold four million copies as of Portal 2's launch -- though that number doesn't take Steam into account.