The Entity Designer in VS 2010 has a number of new extensibility points.  If you are interested in writing extensions for the designer, you should definitely check out the ADO.NET Entity Data Model Designer Extension Starter Kit over on Code Gallery.

When you download the starter kit, you will be downloading a VSIX file.  Save this to your PC and then double-click on it to install a new project template.  Open VS 2010 and choose to create new project based on this template.

There are a number of stub classes that get dropped into the project by default that you can either embrace and extend, or just use as references when adding your own classes.  The project itself has some nice tools to make your life easier, such as integration with the build process to automatically create a VSIX from your project.