Building a Specific .NET Framework Version :: Tip o' The Day

I was working through changing a web project from building to .NET 3.5 to just .NET 3.0.  It was taking a little effort as I removed the .NET 3.5 features.  During that time though I had a problem with System.Web and I realized a cool feature that Visual Studio 2008 has built in.

When one selects a pervious build version, and you try to add a reference, the references that aren't available anymore are grayed out in the Add Reference dialog.