I'm presently working on a Rails project, where I'm discovering the benefits of an MVC framework. So far, I think the clear separation of concerns is definitely the most interesting aspect of it. The point of entry being the controller and not the presentation layer makes it easier to test and makes just more sense. As for TDD, it definitely is easier to see the separation between unit, functional and integration testing. So for me, the biggest advantage of the MVC framework is clarity and the enforcement of a better architecture.
Testing is a continuous challenge for us. But that's good, since it forces us to always challenge the way we do things. As far as implementation, unit testing is not a major issue and for functional testing, we use Selenium. But when it comes to integration testing, this is where our heads spin. Testing against a database is a pain. We've read and heard of many approaches but none seem to be just plain good. For a while we used MbUnit, with the rollback feature, but since then we have moved to an ORM, ActiveRecord. The switch was done recently so I'm still in the process of finding the right way to approach this.
You mention that "MVC could be used for some modules as an option". I'm curious as to what exactly you mean by that. Are you saying that modules could implement an MVC architecture while DNN remain to its classic architecture?
What is your approach to TDD?