You may also want to check out Eclipse (www.eclipse.org).
Its often thought of as Java-centric, but its supposed to be language-neutral, and they have several projects to enable it to support other languages (http://www.eclipse.org/tools/index.html) using its plug-in architecture. They also just came out with a UML toolkit that you might be able to leverage (http://www.eclipse.org/uml2/)
