In my definition, Seam is the universal middleware stack at Red Hat which is most definitely independent of JSR-299 implementation. Therefore, it's my position that anything that "extends" 299 towards "rich web" should be a module of Seam. Extensions which don't go in that direction, such as the Java SE support, are a better fit as a webbeans extension. Furthermore, I don't think you should feel like you "have to depend on Seam" because it isn't going to be like Seam 2 where it is all or nothing. Modules are starkely independent and can be seen as a lose collection of extensions.
Perhaps others will disagree with my definition, or think it too liberal, but if we don't define where Seam begins, then you could easily rationalize all modules of Seam be extensions of Web Beans...I will admit though that the bridge is a gray area in that it could go either way.