Seam is built using JDK 5 features. IBM is a little, er, more patient than their customers regarding JDK 5, and so decided to wait until their Websphere 6.1 release to start using JDK 5. So unless you're deploying to WAS 6.1, you're not going to be able to use Seam.
Wow this is interesting. So eventhough I use JDK 1.5 to run the WAS 5, I still won't be able to run Seam apps?