Somewhere between beta1-tr and beta-2 of Office 2007, the extensibility model of the Ribbon has been changed.
Before you hooked up an event called ThisApplication.ServiceRequest, with which you can register a custom Ribbon.
This time around you need to go to the ThisApplication class and override the RequestRibbonService method.
One thing about this new model that strikes me as odd, is that Word will call RequestRibbonService before the Startup event is raised, meaning you can't use the ThisApplication_Startup method to actually create the Ribbon, but you'll need to use the constructor of ThisApplication instead.