You know what the iPad reminds me of? There was a feature on older Macs, circa System 7, that restricted users to a simple panel of applications and files, preventing them from getting into the guts of the software. (It was used primarily in schools and for "kid-friendly" home computers.) The iPhone, and now the iPad, seem to be run according to the same principle.
Not that that's a bad thing: The iPhone and, by all accounts, the iPad are beautifully designed and capable of making everything from video to web browsing to reading e-books a blast. No one should expect anything less of Apple, after all. But I think a lot of the anxiety you hear in those criticizing the iPad (e.g., Mark Pilgrim) comes from a fear that Apple is leading the charge toward the infantilization of the average computer user: The iPad, among other sins, is locked down and restricts the users from adding any applications that don't have Apple's approval; nor can you access the core functions of the iPad via the command line, as you can on OS X.
But how different is this, really, from the situation for the average computer user today? From the beginning, Apple has been all about making computers and devices "for the rest of us," and the progress of computer technology over the last 30 years has been in line with that mission -- hiding the complicated and just giving us the simple. And it's been wildly successful: Whether you use a Mac, Windows, or Linux machine, a GUI is now the standard manner of interacting with a computer; moreover, most people, I'd wager, only use a few applications, mostly pre-installed, in their daily use. To be sure, Apple is being overprotective in embracing, for the iPad, the walled garden model that it has used for the iPhone. But fears that the iPad's arrival spells the end of the culture of hackers seems, at this point, overblown. Hackers and tinkers are already a minority among computer users, anyway, and I doubt the iPad will exacerbate that dynamic.
On the other hand, the folks defending tinkering aren't wrong, in the general sense that it's somewhat disturbing that so many people in our society are using devices that they neither understand nor care to understand. We're all guilty of this, in one way or another: John Gruber's comparison of the iPad to the automatic transmission may well be correct. Not knowing the intricacies of a modern car doesn't impair one's ability to function in society, any more than not hunting for or growing one's food does. But simply being aware that the things one considers conveniences are in fact the products of generations of effort, ingenuity, and expertise is, I think, salutary. At the very least, it helps one avoid the mindset of passive consumption that modern society can fall prey to. Clarke might have been right that a sufficiently advanced technology is no different than magic, but that doesn't mean it's a good thing to accept that as inevitable.