Krzysztof Cwalina announced that Microsoft has released a CTP of its Managed Extensibility Framework (MEF).
MEF is basically a dependency injection framework that allows to provide extensibility for your applications (add-in/plug-in framework). Also, it's a very good way to componentize your application's architecture and make components/layers pluggable and replacible. Here's a very good overview of what MEF actually is with nice examples and excellent comments from readers: http://blogs.msdn.com/kcwalina/archive/2008/04/25/MEF.aspx
Go grab it here: http://code.msdn.microsoft.com/mef. The download contains MEF itself (ComponentModel.dll) and several interesting samples.
The MEF actually replicates what Spring.NET does but probably in more restricted form. Not sure if MEF really supports xml configuration of dependencies vs declarative dependency injection via attributes.
A while ago we announced Visual Studio 2010 and C# 4.0. In case you'd like to catch up and read articles