This is from a new interview with David Satter on the International Affairs Forum:

The other thing that I heard, and that I think is credible, is that they need time to prepare a kind of pretext. It’s not a simple direct invasion of Georgia without any pretext like the one that Saakashvili so generously provided them for the first invasion. That would be difficult because not only would it cast doubt on the legitimacy of the second invasion, it would raise questions about the first one. Now, to prepare a real pretext for invading Georgia, you need to put some work in. For the first invasion, the preparations began months before the actual crossing of the border. It might be the same in this case. I think for the moment they don’t have the pretext that they need and I think that, more than anything else, is what is restraining them.