According to this, Valve's Doug Lombardi has confirmed that console versions of the Orange Box have sold over 1.5 million copies to date, with PC sales of the multi-game title coming in as "significantly stronger".
"I think the Xbox 360 version did just over a million, while the PS3 [released later in December] version did a few hundred thousand copies," he said. "So I think when all is said and done, The Orange Box will have sold about 1.5 million copies on the console, which is great. But the game's PC sales were much stronger... at least a two-digit percentage increase over the console sales. And that includes Steam sales, retail sales, and now individual sales of Portal, Team Fortress 2, and Half-Life 2: Episode 2. So when you take the PC numbers for The Orange Box a la carte, they're significantly higher."