As it turns out, there are nukes in Call of Duty: Modern Warfare 3…sort of.
Thanks to Modern Warfare 3's Private Match Mode, you can now drop a nuke on your enemies, MW2-style. According to the leak, the nuke can only be unlocked in private matches, and it's actually a gameplay mode – the first to reach 25 kills gets the nuke. I understand why Activision has chosen to include this as a customization tool. Not only is it fun, but how "modern" can warfare be without nukes?
From the leaked video, it looks as if the nuke is not a game ending nuke. Rather, it kills everyone on the opposing team once, and then disables their electronics for the rest of the game, similar to an EMP.. According to the comments, killstreaks don't count towards it, meaning you must get to 25 gun kills to activate the nuke. Apparently, it's a hidden 4th killstreak and it works with all strike packages.