Don't worry, I've answered my own question...

Free Gross Motor Development is a philosophy whose principles were developed and proven through years of research at the Emmi Pikler Institute and Children’s Home in Budapest, Hungary. Dr. Emmi Pikler explains: Moving from one movement stage to another indicates internal, neuromuscular readiness. Each movement has its own history of development. Each one is based upon the other. In other words, each milestone, such as rolling from the belly to the back, builds upon skills mastered in the previous stage (p.12).

The main idea behind this philosophy is that babies should not spend huge amounts of time physically restrained in bouncy seats, carriers, swings, and walkers, prevented from being able wiggle, squirm, and roll. In essence they should be free to move. As far as I can see, tummy time is not a contradiction to this philosophy. I think the main conflict with that idea now, is the fact that with SIDS research "tummy time" has become a "developmental activity" and that some mum's force the issue of tummy time even if their babies roll over when they're put down on their tummies instead of leaving bub to their own devices. From what I've read, I personally don't think it matters which way you put the baby down, as long as they are free to move and explore within the limits of their own physical development.

Source: "Peaceful Babies Contented Mothers" by Dr Emmi Pikler