The MS ADO.NET guys have released a tool that's awesome for diagramming your EF Code-First context. There's more to your EF model than just the POCO classes - for example, the mapping fluent API - and the tool takes it into account.
Without further ado, it's the "EF Power Tools" VS extension (Beta 2 at the time of writing)
Read all about how to install and use it on the related ADO.NET blog post
Be warned, the diagram is read-only, but what would you expect when the input for it is your own code?
