Funny you should mention this..
Just yesterday, I was elbow deep in my Mac, replacing my 5160s with 5655. I figured as long as I was doing this, I'd get a 4870 as well (my ebayed sapphire is on the way right now) Since the cables that come with the card are pc based, I decided to go the route of using the optical bay molex (same as PC molex)
First of all, yeah, those are the mac connectors for the GPU. You can see my existing 1900xt connector right between the 2 molex connectors.
You can see in the pic that I took a long molex y-adapter and pushed it through the little hole with the other cabling up into the optical bay. I switched the cable in the DVD-R so that the end of the cable was plugged into the drive, and the middle connector (closer to the front now) was plugged into the y cable. When the optical bay was back in place, I could pull the 2 ends of the y connector far enough to clear the fan shroud, and I should have no probs connecting the Sapphire when it comes.