Fail, fail fail...
And this is why I will never by an Apple netbook/tablet/thingie. With the iPhone, Apple replaced several small walled gardens with one larger one of their own; one so big (once the app store arrived) that most people could ignore the walls altogether. With the tablet however they are producing at least a netbook equivalent (possibly more powerful) and it would appear taking control away. I won't buy a device of that kind of power if I don't have the sort of control I have over my current desktop and laptop computers. I think it will sell in the US, it may do well in education markets, but there is no way I'd buy one for personal use with an iPhone style lock-down.
With regards to the question "who wouldn't want a desktop computer free of malware" the answer is again, for the average non-tech user, if the walled garden is big enough that you can't see the walls, who will care about the restrictions? I would expect most 'hands dirty' developers to be extremely reluctant to go down this path though because of the degree of freedom that is being sacrificed.
This is an excellent example of Apple's core business practice - build a device that does something well, but also has a built in source of downstream revenue. The real question is, can they get away with this level of control in the area of the marketplace they are entering?
Finally, I'm a keen OS X fan but any attempts to move this into the hardware I use will be met with an immediate and irreversible move to Linux. Although hopefully still on Apple hardware. OK, not that hopefully.