Thanks, Mark. That is a common misconception. The other common misconception is that you must use JBoss AS to use Seam. Seam is a general enterprise application framework that can be used on any Java application server or servlet container. The only barrier is getting the configurations right, but that is true of all Java applications (the nature of the beast).
I will extend your comment by saying that you don't have to use JPA to use Seam. You can use Hibernate, or you can just not use an ORM at all (you could use iBatis, albeit with a lot less benefit). You also don't have to use JTA. You can use a resource-local transaction, which basically means you let the ORM handle the transaction boundaries for you.
Seam is extremely flexible. In fact, if you read chapter 4 of Seam in Action, you will learn that you can override any built-in Seam component with your own implementation. In Seam 2.1, you can even develop your own custom deployment handler so you can also register your own types of resources (and infrastructure) during Seam initialization.
Just keep one thing in mind. Seam is a way to define and wire together stateful components. What technologies you use doesn't really matter.