Recently, I had to upgrade some Microsoft Office Word 2007 Beta 2 add-in projects to RTM. Each add-in manipulated Office Word 2007 documents, so the projects contained a reference to WindowsBase.dll, which uses the System.IO.Packaging namespace. When I was working with Beta 2, I used comments in this post to locate WindowsBase.dll. It used to be found in c:\Program Files\Reference Assemblies\Microsoft\WinFx\v3.0. Now, it is located in c:\Program Files\Reference Assemblies\Microsoft\Framework\v3.0, so the references in the projects obviously needed to be updated.
As always, upgrading solutions from previous builds has some surprises... ;-)