Glenn Block & the Guys from Patterns & Practices just released to the public Prism First Drop, this drop includes a drat Reference Implementation (RI) which you can look and learn how to use the Prism.
If you want to learn more about Prism, Glenn block and me are going to talk about it in less then a month at TechEd Eilat.
Note that this is a code drop i.e. not all the functionality that is planned for the framework is currently implemented through you will be able to have a brief look at what you'll get during summer time.
You are more then welcome to download and give us feedback on what you like what, you dislike and if you have other ideas that we might use, not all the ideas will be implemented by p&p, we are planning to open community prism project where with the help of the community we will implement some of the ideas.