A very handy feature most people don’t know about with Visual Web Developer 2010 Express is that you can change the target framework from .NET 4.0 to 3.5, 3.0 or 2.0.
By default Visual Web Developer 2010 will be set to .NET Framwork 4.0 but if you are in a situation where you have one server, possibly Windows Server 2003 that runs on .NET 3.5 and another server, possibly Windows Server 2008 that is running on .NET 4.0. Being able to target these different frameworks is very useful.
Just right click on your solution in your solution explorer in Visual Web Developer and click on Property Pages. There will be a Build option and then within that you can select your target framework.
Just rebuild your website and away you go. 😉