Interestingly it seems Microsoft are starting to get more info on the next release of NAV out into the channel. If you have access to partnersource, you can download a new whitepaper called Developing Solutions for Microsoft Dynamics NAV "6.0". If you don't have partnersource access, you should keep an eye out for a download on mibuso or dynamicsuser.net.
The document includes a description of the architectural changes in Dynamics NAV "6.0", Development Enhancements, Functionality Redesign Needs, Multiple Client Support, Step-by-step Walkthroughs for creating and modifying pages and reports.
The whitepaper is also available on a new technical preview page that includes a video of the new NAV 6.0 features.