From what i see in the "Get started now!" section in the Introduction to Seam page, Seam can be used with any application server and even Tomcat:
Seam works in any Java EE application server, and even works in Tomcat. If your environment supports EJB 3.0, great! If it doesn't, no problem, you can use Seam's built-in transaction management with JPA or Hibernate3 for persistence.
Seam is not difficult to set up at all. In fact, the setup doesn't change from one application server to the next. What changes is what resources you want Seam to use. If you are using Tomcat, you aren't going to have JTA (unless you break your back trying to configure it) so you have to tell Seam to use resource-local transactions. Also, if you use Tomcat, you have to include a lot more JARs (the stuff that is standard in a Java EE app server).
Seam is not requiring a big commitment from you. The commitment is in getting Seam to find the resources you want to use.