Using portal 2.6.4.
We've setup a portlet to display without decoration via the following:
<window> <window-name>Teasers</window-name> <instance-ref>Teasers</instance-ref> <region>teaser</region> <height>0</height> <properties> <property> <name>theme.renderSetId</name> <value>emptyRenderer</value> </property> </properties> </window>