I happy to say that since last week the UML support for NetBeans IDE was resurrected by the community as you can see the official announcement in the NetBeans new webpage. This effort is fully driven by the NetBeans community and if you wish to help to keep and improve this plugin don't hesitate to contact Javier Ortiz Bultrón or send your offer to email@example.com mailing list.
For more details about what is going on, take a look at the project wiki page.
It is one more prove how strong is the open-source community! Thanks guys!