I have seen some threads on migrating off of the web adf but I don't know of any specific links. We are pretty much moving to flex and silverlight based applications as well and only have two adf sites left out of 7. If you are using the sample flex viewer the is not really any kind of direct migration so you will need to configure the viewer using the config files to work with your existing services which is easy since you just have to update the xml config files. It is fairly quick to get a flex site up and running and the sites are much better than the adf sites.
