As promised, Call of Duty will allow players - for the first time ever - to bring forward content they own from one game into the next. In this case, anything you've unlocked or bought in Modern Warfare 2 will be available in Modern Warfare 3.
This includes the vast majority of weapons, store bundles, skins, operators and everything else you might have accrued. The only caveat is that if the content you're bringing forward does not have a basis in Modern Warfare 3, it won't be available. For instance, if you have a skin for a melee weapon that does not exist in Modern Warfare 3, you won't be able to use it.
Activision put together a big blog post with a detailed FAQ that looks at every potential case and confirms whether or not the item in question will carry forward. The takeaway is that, while you can use anything you own in the new game, you won't be able to unlock it in Modern Warfare 3.