I'm using Eclipse 3.3.2, and I can't seem to use Violet as a plugin. I get "The selected wizard could not be started. ... Reason: Plug-in com.horstmann.violet was unable to load class com.horstmann.violet.eclipseplugin.wizards.NewClassDiagramWizard."
Last I heard, the Violet plugin doesn't work in Eclipse due to changes in the Eclipse plugin API. It does work well as a standalone application, though, which is how I use it.
The integration is not very strong when you do run it within Eclipse; the only interaction I've seen is the ability to drag-and-drop a class to the class diagram and it will create a Class picture for you with attribute names.