There are stores that are, have been and will be selling Modern Warfare 3 early. If you managed to get hold of a copy, good for you! Just be warned, there might be a downside to your early purchase.
Microsoft's Stephen Toulouse has hit Twitter to warn those already in possession of the game that neither Activision nor Microsoft has cleared the title for play on Xbox Live. This means that anyone attempting to play the game online, or even play it offline on a system connected to the internet, runs the risk of having their account reprimanded.
The Xbox 360 has a system in place to stop people with unauthorised early access to games (or pirates) from playing a title early. Reviewers, for example, sometimes have to have their gamertags exempted from searches designed to suspend accounts playing a game ahead of its release.
Not that this will automatically happen to anyone and everyone playing the game now (he only says "may", not "will"), but as you can see above, you have been warned.