It was there, at one point, but with it now totally missing, people are starting to wonder where it's at. Kotaku's Michael McWhertor has heard from a source that GameStop has pulled the game out of its system because the game supposedly requires users to install Valve's Steam client, and GameStop might not feel so hot about selling something that installs a pretty solid digital distribution service that might prevent users from returning to GameStop to spend more money.
Of course, none of that's confirmed. Chris Faylor over at Shacknews seems to think this whole story sounds pretty unlikely, considering the online retailer seems to have no problem selling The Orange Box or Left 4 Dead, both of which also require a Steam installation. Put in those terms... yeah, it does sort of sound like a stretch.
So maybe it's an inventory glitch? Or maybe since this is a THQ game and not a proper Valve release that we're talking about, GameStop is playing super-duper-hardball to send a message to other third-parties: don't partner up with Valve if you want us to stock your game. None of the parties involved in this love triangle of retail excitement have responded to any requests for comment yet, so as of now, it remains a mystery.