You can't patent multi-tasking as an idea. Were the iPhone to get multitasking with the next iteration then the device, and its users, will be all the better for it.
Just like if the N1 were to get multi-touch, even though the largely held rumor is that Google doesn't want to risk getting into a lawsuit due to certain multi-touch patents that Apple does currently own. Although as long as Google creates their's differently there should be no problem.
It all kind of boils down to the whole Pre vs. iPhone thing. When the Pre first launched Apple said it was "looking into" whether or not they could sue Palm to not have multi-touch in their device due to them owning certain patents. Well, as it turns out, Palm's method of multi-touch is different from Apple's, it's just the outcome is the same. Apple is unable to patent the outcome of what multi-touch does, they can only patent their methods of providing it.
Google, Palm and (I think) WinMo all have multi-tasking, but so long as Apple doesn't directly copy each's method of multi-tasking there should be no lawsuit.