I haven't used Seam under JBoss, so I don't know how it works under jboss,
but it works fine under Tomcat, Glassfish, OC4J and Weblogic.
JSR-299 (web beans) is getting ideas of Seam and other JSF frameworks (like ADF) and standardizing them.

the main idea of Seam is started from using EJB-3 session beans as JSF backing beans.
it is better you ignore this feature in enterprise  applications, which are going to support multiple UI technologies or you want to apply separation of concerns principle for better code quality.
there are lots of other features I don't find useful like Spring integration, that is only for helping those migrating from Spring to Seam.
but I prefer seam to other Java EE 5 web frameworks I have used.

sites like Amazon and E-Bay are extremely old and using very old technologies (both are using Java)
Seam is very young compared to these kind of sites.

I have tried IceFaces,
It is more a set of component than a framework, (although it has kind of framework internally for supporting ajax)
I don't like the Ice Faces API at all.
the design has some bad flaws, for example if you want to have multiple select in table, you have to add a boolean field to your row objects!