I've seen that done very often now. But normally done on a painted black cabinet base or a accent color not wood finishes. But then the back splash needs to coordinate well to bring it all together.

You generally want the granite in the kitchen to flow and be consistent, you might like to think about putting the dark granite as a splash back? Or changing the colour of the cabinets

How about posting the surfaces you are talking about?

There are examples of this on houzz so you can get a visual. Have you searched kitchens?
http://houzz.com/photos/87707

Depends on the size and layout of the kitchen.

I see that perhaps black granite with pure white granite in the island might work. My kitchen is part of a great room, with an ample "breakfast room" with cathedral ceilings. Very much like this room:

Yes, you can definitely use two different countertop finishes, but you have to be very careful mixing granites. I agree with mafi28 that probably the only way this would work would be to use Absolute Black on one and then something with a dark/black fleck or veining in it on the other. Consider using whatever black granite you want with the white cabinets and then a white quartz (or Quartzite, which is a real stone) on the island.