Multitasking in the UI is nice on a smartphone, because it lets you listen to one app while viewing another, as well as allowing background monitoring for e.g. chat messages. It's not essential for most users though, since the screen on a smartphone is only big enough to let you view one app at a time, apart from pop up alerts. I would like to see multitasking in OS4 for the iPhone, but I can live without it. The iPad is a different story, the screen is large enough to let you view multiple apps at once, so multitasking makes perfect sense. The demo of iPhone apps running at true resolution, with a big black box around them, was a little embarrassing. Consider, for contrast, if the UI allowed multiple iPhone apps to be running in their own windows, so you could have several on the screen at once. That would be kind of nice. I wouldn't be at all surprised if OS4 allowed 3rd party multi-tasking, even if only on the iPhone.
Having said that, there is absolutely no relationship between multi-tasking and the number of processor cores the device has, except that more power is better for multi-tasking set ups. You can multi-task on one core, and you can run a single multi-threaded app on multiple cores. Of course the iPhone is always multi-tasking, because in the background you've got the daemons that listen for phone calls, text messages, emails etc. As repeated over and over again, the only limitation on multi-tasking in iPhone OS is that the UI doesn't provide a mechanism for opening multiple applications simultaneously. (I would guess that the OS forces 3rd party apps to quit when the home button is pressed, whereas most of the built in apps are allowed to continue when the home button is pressed, the iPod app being the obvious example.) Hard to see how you could get an elegant interface for multi-tasking on iPhone OS, though I'm sure Apple will manage it eventually. Perhaps replace the tray at the bottom of the screen with something more akin to the MacOS dock. The Palm WebOS solution relies on the fact that WebOS apps are web pages at heart, so the card metaphor works okay. It would be a different level of complexity for native compiled apps. Android relies on the availability of a task manager app to force quit rogue applications, hardly the Apple way.
That BSN article is specious bollocks, frankly. Of course the A4 is an ARM based chip, it couldn't run iPhone apps otherwise - unless it's massively more powerful than any other chip of the same size and is able to do virtualization, which I think we can discount. Beyond that BSN have got nothing. To quote their update:
We were told that the ARM licensed its CPU and GPU technology to Apple. That's it.
My guess is that the A4 has four ARM CPU cores, and that they'll produce an A2 with two cores for the next iPhone. However, that's pure speculation and about as badly informed as the BSN article. I would say though that the iPad is going to have somewhat higher processor demands (CPU & GPU) than the iPhone, to allow software like Pages and to drive that five times bigger display. A plain dual core Cortex A9 probably wouldn't show the kind of speed that everyone is attributing to the iPad.
The prospects of MacOSX running on an A4 type chip any time soon are slim to none. To do so, and to allow people to run their existing OSX apps - the only point in doing it - would require virtualization. Apple have done this twice before, with the move from 68K to PPC and from PPC to Intel, but it requires the new processor to be somewhat more powerful than that being emulated. Impressive as the A4 sounds, it's going to be somewhat less powerful than the Intel Core families. Running OSX on it would feel like wading through treacle I should imagine