For those who may have missed it and are moving into .NET 4, Prism 2.2 just released a couple days ago.
You can get a copy from the CodePlex site: http://compositewpf.codeplex.com/ for now, although it should make its way to the MSDN pages on Prism as well.
Functionally there is nothing new in this release. It is really just to have an officially released, and more important, tested and supported version of Prism for .NET 4.0 and Silverlight 4.0 released versions. The WPF version (desktop) is still set to target .NET 3.5 so you don’t have to migrate to use it, but will work fine with .NET 4.0 as well if you change your target framework in the project settings if you want to compile against 4.0.
You’ll also notice that the Really-Big-Long-Name Composite-Application-Guidance-For-WPF-And-Silverlight is fading away. Not sure what legal hoops the team had to jump through, but its just being referred to as Prism now. Long live the community will! I’m sure there will still have to be some back referencing to keep the names tied together, but as I understand it, the next release, which will be called Prism 4, will be called just that.
I’ll be blogging more about Prism 4 as the features gel a little more. For the most part, if you are just getting started with Prism, you can safely start with 2.2, and you will be able to leverage new features as they are added in Prism 4. A couple of the major thrust areas are in providing more guidance around the MVVM pattern and implementation and integration of MEF as an alternative to Unity as the container for modularity and dependency injection. But the MEF pieces will be added on as an option. If you write your app against Unity now, it will still work with Prism 4. And if you want to move over to MEF, it would require some changes in your module code and definitely in the way you do your dependency injection, but is not a substantially different model.
You can read a little more about where things are headed here: http://blogs.msdn.com/b/simplifyingpatternsand_practices/archive/2010/03/15/prism-a-look-ahead.aspx
Additionally, keep your eyes on the codeplex site about. Starting soon you will see public drops of the code in work for Prism 4 including some of the QuickStarts.