I did a quick (really quick) hack to use the jboss templates.
http://jongriffin.comI copied the css verbatim so nothing is optimized, and some doesn't work well.
Some things I notice:
1. There are many useless tags.
2. It is designed for portals so all the themes have a "portal" feel to them.
3. They still use tables.
I think that having a package will do the job for theming. My thoughts on this are to have each subsite query a table that shows what themes are available in the file/db system.
It should be relativly painless to make <master> a data driven parameter. This will make it easy to also make admin pages not themed. This becomes important when you want to "inject" certain elements to some pages (i.e. google analytics, digg etc.) and not others.