As promised earlier in the month Apple has posted a revised iPhone non-disclosure agreement. Two agreements appear to be in place, one is a public agreement (PDF) the other is only available to registered iPhone developers (PDF).
The new NDA continues to protect "Apple Confidential Information" including "any Apple pre-release software (including related documentation and materials) and any information disclosed by Apple to you in connection with Apple Events or Paid Content." Paid content is defined as "certain proprietary content (including, without limitation, video presentations and audio recordings) that Apple may make available to you from time to time for a separate fee." But the NDA now allows developers to collaborate with each other and conduct training and offer books and online guides on using the iPhone SDK.
On a related note Apple has also opened the official iPhone developer forums.