Even if given days of preparation to draft a written response, I don't think I could have answered this question more clearly than Pete Muir, lead of the Seam and Weld projects, does in the following interview. This interview was given by Java Magazin author Michael Schütz at JUDCon Berlin 2010.
Seam 3 interview with Pete Muir
I pulled out a couple of quotes to highlight here, but there's plenty more where they came from.
On the perspective Pete brings to Seam:
I'm a great believer that what makes a good framework developer is really understanding the problems the framework solves.
On how Seam 3 related to CDI:
Think of CDI as the core of Seam 3 - it's the basic programming model for your application components.
On how Seam 3 is structured:
Seam 3 is implemented as a set of portable extensions (modules) for JSR-299 that run in any environment which supports JSR-299.
On community participation:
We encourage contributions from anyone in the community who wants to develop a new module for #Seam 3 (just get in contact!)
I'll add that I thoroughly enjoy working with the community and seeing the exhilarating impact a great idea can have on the group.
On the prospect of a CDI extension directory:
We also moderate a social bookmarking group where anyone can link to a #CDI extension.
On life after JSF:
Coming up, we'll start to see GWT really take its place as a first-class option for writing your view layer. The GWT integration for Seam 3 will focus heavily on integration with Errai.
A lot of pieces had to come together for Seam 3 to take shape. Those pieces are finally aligning and, now that the source has been switched over to Git at github.com, things are going to get heavy.