The south beach diet was written by a heart doctor (I forget his name) and it is not the same as Atkins, but is (like many diets these days) lower in carbs, especially the white simple carbs--white flour, white rice, sugar and so on. And, his thing was that you would lose unhealthy belly fat (this fat seems to be worse for causing fat-related diseases) via the south beach diet.
This Flat Belly diet is in this month's issue of Prevention magazine and it's nothing really new, they are just presenting it a bit differently -- that eating good fats (certain oils, like olive oil, nuts & seeds, avocado, olives & chocolate) at every meal can help you lose belly fat, along with consuming fewer calories and eating every four or five hours.
They've put together some success stories, written a book & put up a website, as most people who come up with a new "diet" do.
Especially Prevention--they are a decades old company, reputable and to my knowledge, reliable and they make their money by bringing ideas to the public that it could take your doctor forever to mention to you (if at all). I've been reading/subscribing for years and years. But, you can find out the info without joining the website promotion or buying the book or even buying the magazine.