Yes, Tomcat replaced Jetty as the default web container in Jboss.
This happened quite a while back, say jboss-3.2.1 or jboss-3.2.3.
Acutally, to be more precise, Jetty is no longer supported by Jboss, but Jboss can still be configured to use Jetty instead of Tomcat.
This is thanks to the Jetty developers keeping Jetty compatible with the latest Jboss releases.

e.g. see http://prdownloads.sourceforge.net/jetty/jetty-5.1.4-jboss-4.0.2.sar?download

If you run into any problems, be sure to search the Jboss forums and Jetty mailing list archives.
FYI, we use jboss-4.0.2 with jetty-5.1.4 without problems after applying a couple of small fixes outlined here:
http://www.jboss.org/index.html?module=bb&op=viewtopic&t=65987

Good luck.