The PCB has provision for 8+6 pins, but that's very optional. Manufacturers may choose use a 8+6 pin design, if they feel it helps their factory-overclocked models. You will note that the reference PCB for GeForce GTX 285 has provision for 8+6 pins, though the product ended up with only 6+6.
The RV790 is a 'reworked' RV770, not an RV770 with higher clocks. So when one says 'it's just an overclocked HD 4870", it should be taken in context of the point that both GPUs have near-identical specs (so far), and not literally.