NOTE: the OSCE has published a reaction in which it takes distance to the Spiegel article. It is my impression that the Spiegel article is the fruit of a talk with someone from OSCE and that that talk includes both own observations and hearsay. However, for some of what he said there was no hard evidence and for that reason the OSCE takes a distance. There may be more trouble for the OSCE: South Ossetia is making the reproach that it should have reported the Georgian attack preparations. On 25/9 the Spiegel wrote an update
Der Spiegel has an article about the road to war in Georgia. According to the article - that is mainly based on OSCE sources - the Georgian attack was well-prepared and was based on a plan made in 2006. Georgia had amassed 12,000 soldiers at the border before it attacked and hoped to easily conquer the 500 Russian and 500 Ossetian soldiers in the Tskhinvali area and occupy the whole of South Ossetia in 15 hours. The article is not very clear why this failed but it notices that the Georgian soldiers were unexperienced (what should also explain the high civilian death toll), Russian air superiority and the speed with which the Russians got reinforcements. Also problematic was that the Georgian attack was led by the Interior Ministry (instead of Defense).
On the diplomatic front the Russian Deputy Foreign Minister Grigory Karasin tried to phone Saakashvili on 7 August. When Saakashvili couldn't be reached he called Daniel Fried in Washington who told him that "Washington was doing its best to get the situation under control". Fried said later that he told the Georgians that they couldn't win from Russia.
There is one claim in the article that bothers me: it claims that the Russians "The soldiers destroyed key bridges, railroad lines and roads" while retreating. In fact the Russians seemed eager to save the infrastructure. Two bridges were destroyed but the Russians deny that they did it and in both cases it may very well have been done by the Georgians in order to hinder the Russian army. This makes me wonder how accurate the remainder of the article is.
Postscript: on 6 november the NY Times published an article "Georgia Claims on Russia War Called Into Question" that confirms the Spiegel reports.
Postscript: in this article from 2011 it is confirmed on basis of the Wikileaks documents that the US knew Georgia was lying about how the war started.