Check this out: http://docs.jboss.com/jbportal/v2.6.2/referenceGuide/html_single/#d0e9773
I've only been using JBP for a couple of months, but from what I've found out so far rendersets dictate how window decoration (titlebar, min, max, etc) are drawn.
In the example on that page you'll see emptyRenderer is used - this causes the titlebar and buttons not to be displayed.
Got it. thanks