I'm very, very surprised that you can't find this documented in the blogs of anyone one the Windows Presentation Foundation team, so here's the tip:
If you have existing Windows Presentation Foundation (Avalon) projects, and try to open those in Visual Studio on a machine that has the July CTP of the .NET Framework 3 installed, Visual Studio may refuse to load them. The immediate cause of the problem is that the July CTP does not include the Visual Studio Enhancements for the .NET Framework 3.
The workaround is this: find a WPF project among the SDK samples (I used the ColorPicker sample for a dll project, and the UsingImageBrush sample for an exe project), open the .csproj file in Notepad, and copy all of the PropertyGroup elements from the sample project file and paste those over all of the PropertyGroup elements in your original project file.
Development goes faster because you don't have that Orcas designer taking about 30 seconds to say, "Whoops!" every time you try to edit your XAML files 🙂