Ben Hogan has a an interesting post about the compatiblity between having a rich domain model and a layered architecture.
I agree with Jon Tirsen on this. It's not practical.
I also agree that a web application does not need a layered architecture. The problem is that if you want to reuse your domain model in a different scenario (like a rich client app) then you'll need the layered architecture, and your domain model will be pretty much useless, or it will require a significant refactoring.