Silverlight blueprint is source code and guidance available for SharePoint developers. It makes it possible to use Silverlight and SharePoint together in business applications and internet websites developed with SharePoint. You can use Silverlight user interface elements in your Web Parts and gives an whole new SharePoint experience to the user.
There are samples, screencast videos and Microsoft Visual Solutions available. The product is also gonna be released as open source on CodePlex within a few months.
http://msdn2.microsoft.com/en-us/sharepoint/cc303301.aspx
Update! Patrick Tisseghem has written a handy checklist about How to configure your SharePoint extended IIS Web App for working with Silverlight 2 applications. This checklist has some usefull information with regards to using Silverlight 2 configuration.