iOS 6 itself is not under NDA. If it were, Apple would be implicating everyone by announcing it publicly at WWDC...
Most development content and APIs related to the iOS 6 SDK, however, are under NDA as long as the SDK hasn't been released to the public developer community. Only certain features which have been publicly announced or are available for public viewing on Apple's site are individually not bound by NDA. Theoretically these topics can be covered, but it's painfully easy to venture into forbidden territory, which is why it's generally advised that we keep all discussion of iOS 6 development to the Apple Developer forums. Besides, if anyone is in the best position to answer questions about iOS software that is in still in early-stage development and testing, it is Apple itself.
We don't forbid questions about iOS 6 or any other development that concerns NDA-bound content per se; in fact, we are not responsible for a developer violating their NDA at all, because the developer is the one who signs the agreement, not Stack Exchange. It is the developer's responsibility and discretion to know what to talk about... and what to keep under wraps.
That said, most questions on Stack Overflow relating to code not working in this beta or that beta can be safely closed as too localized, and that's how we already handle most questions about beta platform development. Just remember that "too localized" does not mean "is bound by NDA and shouldn't be discussed here".