A foreboding, backchannel warning from Xbox Live's top cop seemed to suggest that those who legitimately bought Call of Duty: Modern Warfare 3 from a retailer, before its official street date, could face sanctions if they played it online. Activision has sought to calm those concerns, although still, it kinda sorta would like for you to wait to open your presents and not spoil the surprise.
Here is the official word, verbatim, from an Activision representative to Kotaku.
We value and appreciate the community's support. We have no interest in banning legitimate copies in these last few days. We just ask for everyone to wait for Tuesday when everybody can play the game together.
Read into that what you will, but it sounds like Activision knows it can't vet who should and shouldn't have an actual retail disc this close to release. Unless you have actually pirated the game and are sending info back that suggests that, I wouldn't worry about looking at what's under the tree before midnight.