Pretty much every device you can buy with a satellite navigation function, relies on the U.S. built GPS network for coverage. But now this monopoly is being challenged by similar systems created by other countries. The latest iPhone 4S works on the GPS system just like other smartphones, but it also supports the Russian GLONASS network as a secondary system.
This should allow the iPhone 4S to get better results in places where the American system's coverage can be a bit sketchy, and also provides a backup plan if the GPS system is ever down.
Russia's GLONASS system has its roots in the cold war era, but fell into disuse after the collapse of the Soviet Union. Vladimir Putin recognized the strategic importance of the system, and funded a program to rebuild it starting a decade ago. Now the system is back to full strength, and Apple's adoption of it for the iPhone 4S is the latest feather in GLONASS' cap. Of course the fact that Russia recent imposed a 25% import duty on any sat/nav capable device sold in Russia that didn't include GLONASS, probably played a role in convincing Apple to include it.