They just changed it in the new developer agreement, that you probably agreed to if you logged into your developer portal.

It's under "Information Deemed Apple Confidential". "Further, Apple agrees that You will not be bound by the foregoing confidentiality terms with regard to technical information about pre-release Apple Software and services disclosed by Apple at WWDC (Apple's Worldwide Developers Conference), except that You may not post screen shots, write public reviews or redistribute any pre-release Apple Software or services."

AFAIK NDA's can't cover anything that's publicly available. Apple announced Swift at a publicly streamed press event and they have made a Swift language guide available publicly on iBooks. My guess is they want people using it so that if people do start to switch to it when iOS 8 comes out there will be help available on the usual places (Stack Overflow, forums etc.) and people can find bugs.