After 2.3.1.CR1 release in June, we closed the journey to 2.3.1.Final release yesterday.
There is no so many differences between CR1 and this Final release, look at release notes
Mostly documentation issue fixes and due Richfaces CVE in serialization, Seam has upgraded Richfaces to 4.3.3.Final.
Downloads available at https://sourceforge.net/projects/jboss/files/JBoss%20Seam/2.3.1.Final/
Documentation available at http://seamframework.org/Seam2/Documentation