The embedded Jetty should only be taken for what it's for: a simple servlet container. It's only meant to run servlets, you shouldn't even care whether it's Jetty or Tomcat.
If you feel the need to be consistent, and run Jetty 6 because of that, I'd say you have a problem: you should use an external server instead, with your own software (Jetty7, or even a Jetty8 milestone, Tomcat, whatever) for your tests. You can still use the embedded Jetty for fast iterations when coding, but you should run your integration tests within the server you'd like to deploy with (and not deploy the server matching the one you're happy to easily test against).
As to your initial question, there have been attempts last summer to move to Jetty7 (mainly for the support of WebSockets IIRC) but there were issues with classloaders. John 'jat' Tamplin is an active contributor to the WebSockets protocol at the IETF, and that one seems to have finally stabilize, so GWT will probably move to Jetty7 or Jetty8 in the future… provided the classloader issues can be fixed/worked around.