This is a great link I can pass on to the people responsible for maintaining the servers who were worried (hey, I was worried too) about installing a beta version of the .NET framework next to production apps using non-beta versions. (For those of you not already familiar with Scott Guthrie, he’s a a Corporate VP in the Microsoft Developer Division and his blog posts are always great references.)
.NET 4.0 can also be installed “side by side” with previous versions of .NET on the same machine. .NET 4.0 has a new version number for both the framework libraries and CLR engine – which means it runs completely independently from .NET 2.0, 3.0 and 3.5. What this means is that you can install .NET 4.0 on a machine that has .NET 2.0/3.0/3.5 installed, and configure some applications to run using .NET 4.0 and others to run using the older .NET versions (the IIS admin tool allows you to configure this for ASP.NET applications). This allows you to use .NET 4.0 for new applications – without having to necessarily test and upgrade all your existing ones.
via Multi-Targeting Support (VS 2010 and .NET 4 Series) – ScottGu’s Blog.