In an interview with the Kommersant, the U.S. Ambassador to Russia, John Beyrle, confirmed that the Russian response to Georgia's attack on South Ossetia was legitimate. He also stated, however, that Russia went too far by invading Georgia 'proper', and that Russia now has to abide by the cease-fire agreement and stop hinting at regime-change in Tblisi. A small bit of an AP report in the International Herald Tribune:
John Beyrle [...] told the Kommersant Friday that Russia "gave a well-grounded response" to a Georgian attack on Russian peacekeepers, but exceeded its authority by invading Georgia proper.
Ambassador Beyrle has presumably been green-lighted to give this statement, and he has also stated to the Kommersant that Saakashvili acted without the consent and against the advice of the United States in attacking South Ossetia. This signals a certain ratcheting-down of tensions between the U.S. and Russia, and a readiness on the side of the U.S. to come to an accommodation.
The full interview is available in Russian, here.
Late update: Telo notes in the comments that the translation of the AP might be off, and that the relevant statement by Ambassador Beyrle implies that the Russians had a reason to respond, but is ambiguous on whether that reason was completely sufficient.