Yesterday Jensen linked to the Ribbon developer center. It's where we are bringing together all of our content on Ribbon Extensibility. I just wanted to highlight some of the key resources on the site.
Customizing the Ribbon For Developers, Part 1 takes you through the details of building a solution that customizes the Ribbon. If you're new to the whole idea, which you probably are, start here.
Customizing the Ribbon For Developers, Part 2 includes a complete reference for every valid tag and attribute in a valid custom UI file. It also includes a FAQ with answers to common questions. For example, it tells you how to turn on RibbonX-related error messages. (Application Options > Advanced > General > Show add-in user interface errors.)
The site also includes an overview document and a half-hour screencast that details ribbon customization via the extensibility model.
The Beta 2 Schema will also be on the site shortly. For those of you that can't wait for it, I've attached it here.
Finally, the one document I hope every solution author reads is the Style Guide for Solutions and Add-ins. It'll give you a basic idea of how to think about your add-in or solution in the context of the new user interface. The style guide is really about the cliché "with great power comes great responsibility," a quote that appears on blogs.msdn.com quite often.
Please take a look at the documents and let us know what you think. They'll get updated between now and RTM for sure. Are there any particular samples that you would like to see?