I am just having the same problem with JBoss-2.4.3_Jetty-3.1.1 and JDK 1.4 Beta 2. Would love to know a solution.
Got this to work with JBoss 2.4.4 and Jetty 126.96.36.199 beta. All dev code but now I have jsp support when I run with JDK 1.4 beta 2 (you also need tools.jar on the classpath).
Hopefully with the stable release this will remain the same.