So Ivan Towlson has written up a great post on C# covariance and contravariance in .NET 4.0

http://hestia.typepad.com/flatlander/2008/12/c-covariance-and-contravariance-by-example.html

I know that I burnt my fingers on this issue doing some ASP.NET MVC stuff a few weeks ago so I’m looking forward to getting my hands on the 4.0 bits to use in Anger.