As Matthew Flaschen mentions, you can download a CTP of the WPF Ribbon Control from CodePlex. However, it is important to point out that this is currently very incomplete. I tried it out recently and found it to be decent, but rather buggy.
You won't be able to properly use the WPF Ribbon control (to its full extent at least) until .NET 4.0 is released. I'm not sure whether it's included in the current Beta 1, but it should (almost) definitely be part of the core WPF library in the final release. This ought to be much more stable and complete in terms of features. Whether the feature set will match that of the Microsoft Office Ribbon is another question - doubtful, in my mind, though you might expect WPF by nature to provide some sort of extensibility.
Update: Indeed, the Beta 1 release of the .NET Framework 4.0 details (in the overview section) that a WPF Ribbon control is included. Worth checking it out, as I would strongly suspect it is significantly more advanced than the one on CodePlex.