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.
