This is an escalation of long running territory disputes between Russia and Georgia. The current information seems to be that Georgia moved it's troops into one of these disputed areas, Russia drove them out, and then proceeded to move into the other disputed areas along with several non disputed areas.
One seemingly plausible explanation for them taking the non disputed areas may be that they plan to withdraw from them in a peace agreement while keeping the disputed territories. Making this concession could possibly satisfy parts of the international community.
The sad thing is that currently the U.S. can't do much about this situation. Apart from the empty saber rattling from McCain, we can't take any real military action. Even if we're not pathetically dependent on Russia's oil, our military has been run ragged by the debacle in Iraq, and we don't have the man power to deploy a meaningful force.