One of the questions I hear a lot from customers and friends is "My project is going to ship post Vista and thus post-WinFX, shouldn't I want to bypass Windows Forms and use WPF instead?"
My opinion is that it all depends on the kind of application you are writing (i.e. what will WPF enable for your applications or how will it make them easier to implement?), your tolerance for breaking changes in beta and CTP releases of WPF (although if you are starting now that is no longer a big concern) and your tolerance for building on a v1 technology. There is also a cost to the redistributable in terms of size (on CD or to download), time to install and minimum requirements.
It's good to note that "minimum requirements" doesn't just mean the reach to down level OSs (98, Me or 2K) but you can also bet that the first set of XP machines that came out in 2001 will also struggle or not support WPF. Although the minimum system requirements are not known, my guess is that the current WinFX runtime minimum system requirements detailed on the February CTP info page is a reasonable estimation.
That said, I've never had a great story on what was meant by "tolerance for building on a v1 technology"... until now -- Charles Petzold has a great blog entry that tells the story.
Don't get me wrong, WPF is the UI technology of the future and there are a lot of well documented reasons to move to WPF, it's just important to move to it when it makes the most sense for your products and Charles Petzold's post will help you make that decision.