The Crossbow test automation was run on WTT by the Avalon team on the latest build of Avalon, and it's passed and good to be shipped with WPF. What this means is that WinForms controls can be used in the new WPF applications (using WindowsFormsHost), and the new WPF controls can be used in WinForms applications (using ElementHost), without any issues.
I'm now all heads down on the Vista RC1 Test Pass for .Net (Edit: Removed specific dates). The tests that are failing are mainly due to test driver reasons, because of the new security settings built into LUA. Overall, I'm happy with the improvements in Vista RC1 over the previous builds, especially performance, which is now almost equivalent to that of WinXP. I've also installed Vista (though it's the old Beta 2 build 5384) on my home machine, and played World of Warcraft with no issues whatsoever.
Here's the link to download Vista Beta 2: http://download.windowsvista.com/preview/beta2/en/x86/download.htm
And here's the link to more info on Vista, such as system requirements: http://www.microsoft.com/windowsvista/