The normal Apple Care warranty runs for one year, within that first year you have the opportunity and option to purchase the extended Apple Care warranty, which last another two years, so that makes three years of warranty.
Due to the high failure rte of that specific GPU (8600 GT), Apple established a warranty program, which allows users of the MBP with that GPU getting a repair or replacement after the warranty expired (one or three years, depending on the user and the purchase of the extended warranty).
So in shorter words: That GPU (8600 GT) gets you the opportunity and option of getting a repair or replacement in case of a GPU failure even after you have no warranty left on the machine.
But the program will not run indefinitely, so maybe you can ask Apple about it.
Btw, it does not really matter how expensive a product is, as it is mass produced like many products consumed by human beings, and as it is a highly advanced piece of technology, failure is still a part of its existence. It could happen to your 300€ netbook too, you just wouldn't care as much though.
http://www.apple.com/support/products/
EDIT:
I just found this article, which contradicts my statement: http://support.apple.com/kb/ts2377
Maybe use MRoogle to find out more, as this topic (8600 GT warranty issues) comes up every once in a while, there is even a big, long thread about "screwed 8600 GT users".