According to one source, Apple has been testing a new multi-tasking system for OS X that is similar to the quick-app-switcher function on iPhones, iPads, and iPod touches. The multitasking feature will be functional for applications in the background, according to this person.
I think it’s mostly the UI of iOS. A multitouch gesture would bring the list of running apps, like Cmd-Tab but maybe integrated into Mission Control or something like that. Of course Apple is not going to make a desktop OS behave like a mobile one, it would break every app ever.