Among the greatest criticisms of the upcoming iPad, forgetting for a moment the 800-pound gorilla issue of Flash, are a) no multitasking and b) the lameness of pixel-doubled iPhone apps. Pixel-doubling, of course, refers to Apple's plan on allowing iPad users to run iPad apps, which are designed for the iPad's 320 x 480 pixel screen, on the iPad's much larger 768 x 1024 pixel display by doubling every iPhone app pixel in both directions so that the resulting app now measures 640 x 960. That's a clever idea, but an app that looks crisp and sharp on the iPhone may look lost and clumsy in pixel-doubled mode on the iPad. How about app-quadrupling instead?
What do I mean by app-quadrupling? Well, allowing the user to selectively call up four 320 x 480 pixel iPhone apps and display them in a grid. This way, the user could have their four favorite apps open and simply tap one to make it "active." This still would not technically qualify as multi-tasking, but it would approximate it nicely. In essence, the OS would call up an app, display it in one of the quadrants, then either "freeze" it when the user taps on another app, or, if a limited degree of multi-tasking is available, the app may even update itself.
Since a block of four iPhone apps only takes up 640 x 960 pixel on the 768 x 1024 pixel iPad display, there'd be 128 pixel left over on the side, and 64 pixel on the top or bottom, enough to have a horizontal menu and a verical scroll bar for other apps (see mock-up iPad screen). Apps could be selectively opened and closed, and also rearranged. There might even be a way to call up frequently used "quads" of apps.
I don't know if app-quadrupling would be doable under the current constraints of the iPhone/iPad OS, or if something like this is already in the works. If it is doable, I think it would instantly turn one of the iPad's perceived weaknesses (only one app at a time) into a definite strength (quick and easy access to multiple apps or app groups).