I have done several of these, my favorite one being where we moved servers AND switched all of the underlying code at once (including DB schema). Oh, and the traffic was very write heavy, and coming not from the web, but from a large number of hardware devices running Java 1.4 over 2G using UDP. The packet routing for this was complex and involved at least two pieces of home-grown software (and even IP spoofing). I love shit like this, and this article highlights a few cool ways you can do this migration. In this case I would have set up replication to a secondary MySQL server, then used reverse procuring to let DNS propagate and the finally make the secondary MySQL server the master server.
