This is based on an earlier ObjCViz project, by @olg. I just updated it to work with the modern objective-c runtime.
Currently you have to link the framework built by this project into your app, then call -[obj graphvizRepresentation] on the interesting object. A better UI may be forthcoming.
See the example at the original project page above. Notice that this isn't generating class or interaction diagrams, it's a graph representing the current, live state of the object.
See LICENSE.txt.