It will depend of the kind of people will watch your workshop, it they are familiar with ASP.NET, ASP.NET MVC or the MVC pattern, etc.
I think that the first and bigger concept behind the MVC is the separation of concerns, and you should show it and how it can help the developers, specially with the TESTING issue.
If people are not familiar with MVC, only this part would take a good time. If they're familiar and you want to talk specifically about the version 2 of the framework, I think the bigger changes from the previous one were:
- Areas
- Template Helpers
- DataAnnotation Attributes
- Model validator
With few examples, once 3 hours is few time to get deeper in each of these points. Would be nice to develop a few app live so they can see how easy it is.