A quick summary of the changes to the schema that I ran into:
<fileMenu> becomes <officeMenu>
<advanced> becomes <dialogBoxLauncher>
And the built-in Control IDs have changed as well, so you'll have to update your XML to reflect these. The Office team has provided a tool to automate this called UpdateIDMSO.
Savraj Dhanjal, PM on the Office User Experience team helped me through these changes, and has a great blog post that details what is new, including some nice tools to help you out. He also provides a list of all the icons in the icon gallery and how to access them when building your Ribbon UI.
If you are looking to see how easy it is to create a custom UI in Office 2007, take a look at the short screencast titled Extending the Office 2007 UI with a Custom Ribbon, and then follow that up with a visit to the Ribbon UI section on MSDN.