Well, it may not be exactly what you are looking for, but I did recently update my 3.9 build script: http://www.saltypickle.com/Home/18
It presently builds a 3.9a image with my favorite tools and configurations from a brand new image. It builds images with a boat load of my favorite packages in about 15 minutes. I suspect it could be automated to generate nightly builds.
Indeed this would be a good functionality, may be this should be added to the testServer running at: http://testserver.smallhosting.info/
In the context of building a testserver that would automatically take an image and run the tests (and remove or load some packages). one of the questions we had is how can we get a list of the packages to test for example a given dsitribution