The problem with iPhone OS is that (in my opinion) it makes a great OS for small devices but the iPad is a bit too big for it: in both size and potential. Let me explain:
Size-wise, I think it's obvious to anyone that the interface looks exactly like the iPhone's. The App icons are similarly placed without any acknowledgment that you are working on a larger screen with more space. And while smart that it can use iPhone-formatted App Store apps, their use on an iPad will likely make it obvious that they just weren't made for that device.
Potential-wise, I think having such a large device should also lead to more processing potential. You no longer need to factor in the mission critical aspect of taking calls or even music playing as is the case for the iPhone or iPod touch.
This device, unlike the other two, has no distinct, obvious and necessary purpose. If there is no necessary task the iPad needs to preform without fail, why can't you experiment with multi-tasking? With so much more room to play with, you should also expect more computer-like behavior and productivity (ex: the iWork apps), including plug-ins such as Flash and multi-tasking.
Thank you for having something intelligent to say. I would like to respond.
I honestly do not think the iPad is too big for iPhone OS. I do think it is too small for Mac OS. Window management is a mess on Mac OS. Sure, the tools to get to the windows like Spaces and Expose make things more bearable. But having been a Mac user for some time and seeing how other Mac users work. There are layers upon layers of windows. This wouldn't translate very well to a 9.7" device. I couldn't see Expose probably wouldn't translate easily. Spaces, I could see, maybe an implementation like the Pre or Android, multiple screens with apps and widgets. It would take work to translate Mac OS to iPad in such a way that keeps it familiar while giving the user a proper experience.
Now, let us say they've got something that will work. What about the apps? Well, Universal now has a new meaning: PPC, Intel and Apple (A4). This transition will take time, especially for big shops like Adobe. We probably wouldn't see CS until CS5. In addition, developers will have to tweak their UIs to something more friendly for a 9.7" touchscreen. This going to take time.
These two things mean that the initial experience of this new Apple tablet running Mac OS would be incredibly lackluster. You would be limited to Apple apps initially. Remind you of something? The original iPhone san App Store?
Could Apple do this? Probably. They could make it work. But why do that when you already have the infrastructure for such a device? They have a Touch SDK, a distribution model, and apps available. Apple went in this direction and we will have to see how it falls out. I honestly think the developer are what will set this device apart. We saw some of that in the Keynote.
I also think multi-tasking will eventually find its way into the device. But I haven't really seen any feasible suggestions. I really think that is a struggle for Apple. How do you introduce new elements to an established OS without shocking the user base? People are used to tapping on an icon, running that app, hitting the home button and running another. I would love to hear some suggestions of what people desire as far as multi-tasking. People are asking for it and I don't know if they know what they are really asking for. It isn't as simple as allowing resizable windows.
In summation, iPad OS is here to stay on this device. While it appears that it has no distinct purpose, I think this is intentional. The developers are going to determine the direction of this and I think Apple has pushed them in the right direction (productivity, content creation, etc).
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