I have a setup a site for Online Calculators that I created about 2 years ago to get up-to-speed on Silverlight 2. I've been going through and converting all of my web sites to VS2010, MVC 2, .NET 4, and jQuery 1.4.1.
I anticipated that the calculators were going to be a bit difficult because I'd also included some Codeplex Silverlight Futures (or whatever it was called) charting libraries in the original project and because I was also upgrading the hosting site from .NET 2.0 to .NET 4.0 and well as MVC1 to 2.
I was very impressed with the ease of the upgrade. The VS2010 wizard did it all and it probably took me less that half an hour to complete everything from end to end including setting up the web site on a new server.
The part that I was most concerned with were the charts in the Mortgage Calculator because of the origin (Codeplex) of the libraries and that they may have changed for Silverlight 3. However those went swimmingly. The host site is very basic and so the conversion to .NET 4 and MVC 2 was not a great challenge for the wizard.