Dealing with GPL Violations

Sortova writes: "For many years now I've been maintaining OpenNMS, a free and open source network management framework. OpenNMS is published under the GPL. A couple of years ago it came to our attention that a company called Cittio was using OpenNMS as part of their proprietary and commercial network management application. I talked with Jamie Lerner, the Cittio founder, and he assured me that Cittio was abiding by the GPL. However, we were recently contacted by a potential client who was also considering Cittio's Watchtower, and it appears that they are not disclosing that they are using GPL'd code or at least not in the clear and concise fashion required by the GPL, including the offer of source code for all of the code they are including and any changes being made to that code. Since the copyright for OpenNMS is held by a number of commercial companies, the Software Freedom Law Center is not able to help us defend or even investigate a potential violation. I was curious if anyone here on Slashdot had experienced something similar or had any advice?"