If you haven't heard, all of the various pieces you need to do development on WPF are available now for RC1. A few things I wanted to chime in on:
--If you haven't checked out the new Cider, do it! The XAML/Designer split view is worth downloading it alone. It is like XAMLPad but with intellisense, color coding and more. Plus, more and more is successfully being parsed by its design surface.
--There are no breaking changes (phew, now that is welcome relief) although there is one bug that folks should be aware of that is buried in the release notes. It has to do with the tiering API, so if you are counting on the the tiering API and looking to release on RC1, definitely take a look.
--I am seeing some very flaky behavior with Vista, graphics card drivers (both NVidia and ATI) and some WPF applications, especially those that use video and 3D. Symtoms include pretty bad artifacting and even some reported blue screens (haven't seen that personally). If you are seeing any weirdness of this type, I'd be very interested in hearing about the graphics card you are using and what in WPF you are exercising that causes this.