So here is a theory that I have heard. Note, this is not insider information. A friend who is a developer, not on consoles or PlayStation mind, thinks that the new devkits probably ship with 8GB GDDR5 but the developer tools probably take up 2-3GB on there, leaving addressable RAM at just 5GB. He said the problem is that the APU bus is 256bit which won't allow for more than 8GB RAM until 8Gbit chips are available.
He thinks that the current SDK probably does have 3.5GB reserved for for the OS functions and development tools, but when the next set of kits ship they will have 12GB of RAM and the full 7GB will be addressable.