I'm not sure when the pre-built JBoss/Jetty bundle will be released - most likely a time constraint on the developers. However, the source contains the service module so you can install the Jetty module and remove the Tomcat module when you build the distribution. Alternatively, you can get the Jetty distribution from the Jetty SourceForge site, build and install that. The microkernel design of JBoss allows you to just deploy the Jetty plugin module. Refer to http://amitysolutions.com.au/documents/JBossJetty4.2.14-technote.pdf for information on building from the Jetty distribution if you need it.
Hope that helps.