MSDN article "Transitioning Your Existing Access Applications to Access 2007"
There is a good new article by Jan Fransen and Ken Getz that is designed to be a guide for users to take advantage of new features.
Summary: Learn about some of the new features in Access 2007 and how you can take advantage of them to improve your existing Access applications. Read about some of the issues you should consider when converting your databases to the new ACCDB format. (18 printed pages)
You will also want to refer to the article on custom ribbons which I talked about in this blog post.