Richard Baldwin on Free Exchange makes an astute observation about the nature and purpose of Russia's forays in Georgia:
One tried-and-true glory-restorer is sabre rattling, and here the Georgian crisis was nearly perfect. Russia was reacting. Russia was standing up to the West. The hearts of Russian patriots beat fast and strong. Something like the May Day parade but with much better TV footage. Best of all it was dirt cheap, easily worth the boost in legitimacy it gave the government.

If my conjectures on objectives are right, Russian leaders will milk this crisis for as long as they can. Rattle the west’s cage as many times as possible. But they will avoid real war like the plague. Real war—like the kind that the Soviets fought and the kind that Krugman worries about – is expensive and eventually unpopular. Not at all the sort of thing that will keep Putin’s group in power and in the money.
In addition to that, there's also the obvious oil motive. Gazprom has interests in parts of Georgia.