WebBeans represents a small part of Seam -- the JSF EJB3 integration part. WebBeans will be structured around Seam not the other way around (Gavin is driving force behind WebBeans). So, no, Seam is not going to change because of WebBeans.
Yes, Seam is very different from Spring + Hibernate. Actually, Gavin (the inventor of Hibernate) invented Seam because he is frustrated that people are using Hibernate incorrectly with Spring. He contends that the stateless Spring is not a natural fit for Hibernate (all those lazy loading errors!) and hence he started the Seam project.