Microsoft just released Code Contracts, a tool that integrates with Visual Studio and allows you to define contracts for your .Net code and get runtime and compile time checking.
Watch the video on Channel 9 that shows how it being used.
For now it's an add-on but it will be part of the Base Class Library in .Net 4.0
Is this something you see yourself using?
I wonder if this means the death of Spec#?
Update
What I mean by the death of Spec# is that we now have 2 different projects for writing contracts:
Spec# is an evolution of C# and it introduces new keywords and behaviours; on the other hand, what Microsoft just released is a library that can be used with any .Net language.
Since the latter looks like it's going to become the de-facto standard, I wonder where that leaves Spec#