At first, Microsoft seemed to think there wouldn't be an issue, with Xbox LIVE Director of Policy and Enforcement, Stephen Toulouse, writing, "For those asking about MW3 pre-release play: If your copy is legit and obtained legitly, have fun. It's a great game." But soon after, the story changed. He updated his Twitter with a new response, straight from Activision: "clarification: dblchk'd with Activision. Mw3 pre-release play not authorized," he wrote, "So pls be patient. Playing early may impact your account!"
It's a vague statement to be sure, but it's certainly ominous. It's also somewhat insulting – it's really not the consumer's fault in this case. It's the retailers, the publishers, and the employees, but not at all the customer's.
So there's your warning! If you go to the store and see a copy of a new game on the shelf, don't buy it! If you legally purchase that game and play it online, you might be punished by the publisher (who should have done a better job marking the shipment boxes to avoid confusion).