Absolutely. This really came about because it was originally called JBoss Seam, but Seam is more JBoss than Hibernate is. Take a look at http://docs.jboss.org/seam/latest/reference/en-US/html/index.html chapters 37 - 39. You'll find details there about Websphere, Web Logic and Glassfish. Seam can also be deployed inside Tomcat or Jetty and can even leverage EJBs inside those containers using the Embedded JBoss EJB container.
It should also be noted that in SVN, seam-gen can create projects that are deployable to JBoss AS 4, JBoss AS 5, GlassFish V2, and GlassFish V3. That's just to demonstrate what is possible. With just a few extra tweaks to the build, the project could be deployed to other servers as well.
Understand that Seam is, was, and always will be about supplementing Java EE to give you features today that are being worked into the specs of tomorrow. So there is no reason why Seam shouldn't be able to run on any application server because it extends the platform the way it was designed. Now and again Seam developers have made mistakes that have messed up compatibility, both those were just bugs and got resolved by community feedback.