Contravariance and Covariance at last

It seems the information deluge has started, and the first piece of good news about C# 4.0 is the introduction of contravariance and covariance for delegates and interfaces.

Charlie Calvert has a useful summary that I'd suggest you read.

On first glance, it looks as though the support hinges on adding a in and out modifiers to type parameters as hints for further use.

I'd like to know if the compiler will be doing any checking/enforcing in this area - are the in and out modifiers just hints, or will the compiler be doing some semantic checking to enforce the rules.