Multi-JSF is a JBoss AS feature that I've wanted to implement for a very, very long time. Basically, it allows you to use any JSF implementation as if it were the impl that ships with JBoss AS. This works with almost any version of MyFaces or Mojarra. You can even install multiple JSF implementations and tell each app which one to use.
Until now, if you wanted to use a different JSF version, you could hack AS7. But this only worked for Mojarra implementations. If you wanted to use MyFaces, you had to resort to bundling JSF with your web app and setting WAR_BUNDLES_JSF_IMPL in web.xml.
Both methods had potential integration problems if you wanted to use JEE5 features such as @EJB , or JEE6 features like CDI and bean validation. What's worse is that if you wanted to use more than one JSF version you would end up bundling JSF with every WAR.
In short, it was messy and it didn't always work. Plus, WAR_BUNDLES_JSF_IMPL isn't even officially supported.
Multi-JSF is now in the latest nightly build of 7.2.0-Alpha1. You can get all the details on how to use it at https://community.jboss.org/wiki/DesignOfAS7Multi-JSFFeature.
I hope you'll try it out and provide some feedback before AS7.2 goes final.
So long and thanks for all the fish,