Mike Henderlight blogs that the "Crossbow"-technology will actually be part of WPF.

"We have spent the last several months working very hard on developing and executing on a plan where we could deliver the Crossbow runtime as an integral part of WPF.  As of the Beta2 release of WPF, the Crossbow runtime will be part of the WPF redist and reside in the GAC just like all of the other WPF components.  And from that point forward we will be on the WPF ship schedule which means that we will RTM with WPF."

This is great! The Crossbow-technology has been created by a seperate team within Microsoft, but I'm sure when WPF hits the streets most developers will assume this to be a very "oh yeah, there is interop with WinForms, what's new" kind of feature. You know? One that everyone expects to be there, but is really very very cool.