It seems to me the UML module of NetBeans is a bit too much hidden. In NetBeans 6.5 it was very easy to create an UML diagram. No plugin installation necessary or sth. like.
Re
If you go here and download the UML module cluster you can simply copy it into the NetBeans install directory and then NetBeans will have UML functionality again!
If you don't feel like looking through the page of module clusters the direct download link is here.