Prism is the codename for the new guidance for Composite Application development for WPF. This is not an update to CAB (Composite UI Application Block), it delivers a new set of assets and guidance for developing new Composite WPF applications. As it is not based on previous smart client offerings it is really optimized for WPF and gives a simplified approach that is more easily adoptable. Prism includes a reference implementation, reusable library code and pattern guidance.
I've been reading on Prism and collected a few URLs that should get you started on Prism. Please note that this is still not in beta, drop 6 is currently available for download from the CodePlex site.
If you are wondering whether Prism is a replacement for Acropolis (which has been delayed), it's not really the same thing. Acropolis is still undergoing and will be folded into future versions of the .NET platform. "Prism" focuses only on a subset of what Acropolis was aiming to provide.
There is one thing I don't like though, hope the team comes up with a nicer logo :-) sorry guys, really not my thing.
What do you mean by a "pattern guidance"? As in design pattern guidance?
If I'm understanding your question correctly: yes that's part of Prism, it gives you guidance on using design patterns in composite WPF applications.
The Patterns & Practices team at Microsoft released the Composite Application Guidance for WPF earlier