How can the iPhone SDK be NDA'd?

It's not the SDK that you pay for, it's the beta SDK that you pay for (along with cert signing and app distribution). The 3.2 SDK is currently only available for paid developers. Everyone else gets 3.1 and the docs are available without a login. I understand it's a moot point when people are violating the NDA left and right, but it's supposed to be that the information in the 3.2 SDK about the iPad is all confidential info.

It's confidential in theory, it's just nearly unenforceable in practice. It reeks of evil because it's only enforceable via fear.

You're asking how it can be confidential? It's confidential because you signed a contract agreeing that it's confidential.

Sure, if you got everyone who bought your book to sign a contract agreeing not to write bad reviews, then you could potentially sue them if they violate that contract. Indeed I'm aware of several hardware and software products that at least claim that by using them you agree not to post bad reviews or benchmarks.

Actually, I think there is an interesting question here. Please bear with me, because I am not a lawyer and my knowledge of intellectual property law is limited, but here is what I think applies.

(1) There are various sorts of intellectual property, with different standards and rules. For instance, there is patent law: if Apple had a patent on the API of the iPhone then they could tell anyone not to use it except on Apple's terms. They don't have a patent. There are two other sorts of law which might apply: copyright law and trade secret law.

(2) Copyright law would allow Apple to go after anyone caught making copies of the API docs. But it would NOT allow Apple to prosecute anyone for mailing a printout to all their friends, nor could they prosecute anyone for knowing the API or for writing code that calls it. On the other hand, Apple clearly has a copyright on the API docs.

(3) Trade secret law is another possible area that might apply. The archetypal example of trade secret law is the "secret formula" for Coke. To be a trade secret, Apple's business must depend, to some extent, on keeping it a secret (I think we can safely assume this applies). Also, Apple must make an effort to keep it secret -- that's what the NDAs are all about. The point that the article raises is that perhaps Apple's willingness to share the "secret" to anyone with $100 bucks to burn weakens this to the point where they can no longer claim the full advantages of Trade Secret protection.

I believe that one of the things Trade Secret protection provides is the ability to constrain 3rd parties. Contract law allows Apple to sue the guy who leaks the API doc -- but perhaps he's a bum with $100 to his name: there's only so much they can do. Trade secret law (I believe) is what would allow Apple to tell OTHER people who got a copy from this bum to relinquish their copy and stop using the information. If Apple's low price and wide number of customers for the iPhone API have weakened this sort of protection, that is interesting.

It is a simple matter of contract law - it is an exchanged for bargain. In consideration of you getting access to the SDK, etc., you are agreeing to a non disclosure clause and paying $99, among other things. If you don't want to enter into that agreement, don't.

Shrink wrap licenses and related items are generally enforceable.

It's Apple. They sue people for spreading rumours, what do you expect?