"...make it run OS X..."
Putting aside the debate over the closed/open nature of the iPad, I suspect this would be extremely popular with a small niche of users, and overall would be a colossal mistake on Apple's part.
Pretty much all previous tablet attempts that actually shipped have used desktop operating systems for the platform. Pretty much all previous attempts have failed. As someone who had the misfortune of using a Windows tablet for a while, I can tell you that desktop operating systems are clearly NOT MEANT for tablet use. Sure, you can cram touch or handwriting into them, just like someone can put on shoes that don't fit quite right. But the reality is that the experience will always feel sub-par; your feet will hurt with the ill-fitting shoes, and your computing experience will suffer using a desktop environment on a tablet machine. (This applies to OS X, too, if you look at the Axiom Modbook machines.) And I suspect Apple isn't interested in offering a sub-par experience, as previous tablets have. The iPad may be more limited than a 'full featured computer,' but (as someone who's tried this both ways) also feels MUCH more natural to use than a desktop operating system when you're dealing with touch on a tablet.
But moreover, you rightly make the point that 'the Xbox didn't need to act like a PC,' and (whether we like it or not) the iPad is not trying to be the same thing as a desktop PC either. The iPad is trying to be an appliance, like a television or a microwave; something you just use, and don't have to worry about all the things average folks don't want to have to worry about. The simple truth is that techies want their devices open, but average folks don't care. They just want it to work. Even Microsoft's realized this now, which is why the Windows Phone 7 platform is apparently not allowing native code to run (witness the cancellation of Fennec for Windows Mobile), and has an Apple-like app storefront you submit to through Microsoft so they can better control the experience and stability. And while we hacker sorts lament the loss of ability to muck freely with our devices (without having to 'root' or 'jailbreak' or whatever the terminology for your platform of choice is), the less technical sorts are going, "Oh! Now /I/ can use these shiny gadgets, too!"
Most people I handed my old Tablet PC to went "WTF?" and got frustrated. My aunt, who had given up entirely on computers after the hassles she had with her old PC, toyed with an iPad the other day and remarked in surprise, "I could use this and have e-mail again!" The difference is fairly dramatic. The Tablet PC was trying to be a desktop PC stuffed into a tablet, and gave a lot of power to the user but did not work optimally. The iPad is /not/ trying to be a desktop PC at all... and that gives Apple the freedom to throw out the existing usage paradigm entirely, rather than shoehorning the desktop into a touch device.
We can hope they extend the platform and make it more flexible and powerful, but I think we're more likely to see the mobile branch of OS X (iPhone, iPad) expanded out to get new capabilities than we are to see them "make it run OS X" as you suggest. Simply because the mobile branch's usage model is better suited to phone and tablet use than the desktop model is.