It seems the main desire for multitasking in iPhoneOS is twofold: (1) Running background processes, either to do something else like Pandora music (the #1 complaint I've seen mentioned) or to update other apps like Twitter or Facebook. (2) Relating to information in one app from within another app.
Putting aside the first issue, which has been half excused with push notifications (though it seems like a hack) the second issue has perhaps even a better solution than multitasking.
If iPhoneOS apps could publish controls that other apps could access then a user's experience could be more seemless and smoother than the current multitasking experience in Windows or OSX.
Imagine for a minute, if you want to quote a mail you recently received, you perform the 'include' gesture and you are presented with a popover or modal screen presenting you with a choice of the 'include' controls published by your other installed apps. You can click the include mail control, and be presented with a list of your mail to select one or more messages and 'include' them in your current app. The means by which they are 'included' depends on how your current app chooses to deal with included material.
This is essentially what the many image manipulation programs do now when you go to your photo roll to select an image to edit. In addition to being more seemless and more intuitive from a simple user workflow, we've eliminated the concept of files and replaced them with resources belonging to one app or another.
Similarly you could provide 'peek' functionality to just look at another app (at least the info they make available through their peek control) for a moment without leaving your current app. I'm sure there's probably six main types of controls (including 'include' and 'peek' mentioned here) that could create a standard visual and user-flow language that would answer most of a user's multitasking needs in a way that is better than multitasking.
The controls have to feel very responsive lightweight and fast, of course, but the benefit is that the line between the OS and the apps is blurred in terms of who provides functionality, but clear in that it is apparent when you are using an app's controls, and apple still has ultimate control to ensure the user experience doesn't suffer. You could even use the 'lift up the app to reveal a page behind' interface we saw in the map app in the iPad demo as the default place for the user to choose one of the six control types, and then be presented with a list of controls.
I for one would welcome an iPhone/iPad app-control-oriented future instead of a multitasking future. Users won't be able to lose windows, or leave tasks running in the background which they have forgotten about. It's a serious win for the 80% users.