Framework Design Studio On MSDN Code Gallery
I just came across this today:
Framework Design Studio is a set of tools for reusable library designers. The package contains a GUI tool for viewing, reviewing, and comparing versions of managed APIs. It also contains a command line tool for generating API diff reports.
This seems to be a great tool to check if your new version of your framework will break any previous released APIs.
Although honoring the previously released APIs, your new version might have side effects and, to find out what they might be, there is another great tool: NDepend.