Read a blog post by Dino Esposito The dead-end of Web Forms which is where Dino says:
When introduced, Web Forms was a cutting edge solution and it just engineered current best practices. But it was ten years ago. We could argue whether it was the right choice to engineer ASP practices ten years ago. There's not much more you can expert or achieve with Web Forms than you do today. OK, tomorrow, with version 4. This is the dead-end of Web Forms.
Does it mean that no more development will be done on web forms by Microsoft, I doubt it, will there be a lot of new development, that depends.
Web forms is a mature product this means that any changes to it will be incremental amending/enhancing existing functionality to improve how it works rather than jumping forward with innovative new functionality.
If you look at the changes in .Net 4.0 we see some of the new functionality being introduced having already been developed for MVC, for example routing, and I think going forward you will this trend increase with new functionality appearing in MVC first and then being added to web forms afterwards.
If Microsoft themselves believe that web forms has hit a dead end then why not open it up to the community as open source the same way as MVC, if this happened at worst no changes would be made by the community but at best you may see new innovation and the product revitalised.