I found one day when I was looking at the 8800GT for the Mac Pro that their is an area for a SLI connector. I was thinking about buying one for my Hackintosh.

I know that the ATi Radeon HD 2600 XT doesn't use a chip bridge of any kind and Mac simply recognizes all the graphics cards as one.

What is the reason for this if Mac OS X can't use Crossfire/SLI and that the SLI thing is there?

Here is the link, look through the pictures, and you will see it:

http://www.nvidia.com/object/geforce_8800gt_for_mac.html

Very strange...

OSX doesn't use the crossfire capability. When in windows it will enable software based crossfire support.
Due to the lack of an Nvidia component on the motherboard inside the Mac Pro Nvidia doesn't allow the use of SLI on the machine.
The SLI connector is most likely there because there is no point to redesign the card to remove the port when it can just be disabled in the EFI firmware on the card that was written for the Mac.