There are currently three tools - foreign relay, master_sync and fkie. Some of us on the multimaster sig are working on putting together a new gateway model (about 40% complete).
Best advice is to check the multimaster sig for more information and to track development.
master_sync does directed flipping. It can flip specific local topics/services to a remote master and is configurable from yaml or from code. It doesn't work on fuerte though. rocon_master_sync is a patched version of master_sync for fuerte. rocon_linux_app_platform makes use of it and turtle concert is a working demo.
Master sync though is only two-master. To scale up, we're in the middle of creating a gateway model for flipping/requesting topics between peers in a multimaster environment. It's very crudely flipping publishers and services right now and we're targetting for it to be reasonably polished within a month.