Basically, if you look at the architecture of any modern GPU, R5xx/6xx or G80, it comprises pretty modular units connected by a big interconnect. Imagine if the interconnect was more distributed like say an Opteron and HT, you could have four small chips instead of one big one.
This would have massive advantages on design time, you need to make a chip of quarter the size or less, and just place many of them on the PCB. If you want a low-end board, use one, mid-range use four, pimped out edition, 16. You get the idea, Lego.
It takes a good bit of software magic to make this work, but word has it that ATI has figured out this secret sauce. What this means is R700 boards will be more modular, more scalable, more consistent top to bottom, and cheaper to fab. In fact, when they launch one SKU, they will have the capability to launch them all. It is a win/win for ATI.