Seam 1.2.1.GA was just released. There is one very important bugfix to be found in the release notes:

[JBSEAM-996] - EntityManagerProxy should be serializable

If i´m right about the implications of this one, this might have been responsible for the exceptions i´ve seen (and not been able to track down) when JBoss AS 4.0.5 tries to passivate Seam Components.