The February CTP of the WinFX Runtime Components is now available. The big change this time around is Windows Presentation Foundation which benefits from several months of development work (Nov, Dec, Jan didn't really include any significant changes).
Importantly the API is now complete and we are not expecting any major breaking changes. Hence now is a great time to start working with WPF. Check out the following links:
Please note: the current version of the Expression Interactive Designer does not work with the Feb CTP. Expect a new release soon.