How can I force Visual Studio 2010 to use MSBuild 3.5 instead of the new 4.0 ?
You can target different versions of runtime from the project properties dialog:
alt text http://img14.imageshack.us/img14/3051/vstargetframework.png
Not sure if modifying the build rules to use MsBuild 3.5 for C#/VB.NET targets is a good idea.
(The below change is unnecessary. Using .NET 4 targets file has no problem at all.) Edit Try to open the c# project with a text editor and modify this line:
to this: