may remember, Ubisoft's Assassin's Creed was the first game to have some use for DirectX 10.1, at least part of it. But as soon as that was discovered, as it was the "hidden" feature of the Assassin's Creed, Ubisoft has issued the famous 1.02 patch that has removed this feature.
Our colleagues over at Pcgameshardware.de have done an e-mail interview with Ubisoft, and tried to get some info on this situation. You may have heard the explanation for the DirectX 10.1 support in Assassin's Creed before, but just for the sake of the argument, we will do it once again.
According to Ubisoft, they have found that they could optimize a pass by reusing an existing buffer, which is one of the great features of DX10.1 API. But unfortunately, at least according to Ubisoft, the performance gains that were observed in the retail version were inaccurate, since the implementation was wrong and a part of the rendering pipeline was broken.
This was only notable when anti-aliasing was pushed into play, but in any other scenario both Dx10 and Dx10.1 use the same rendering pipeline. According to that same interview, the mentioned feature was buggy and Ubisoft had to remove it.
When asked about the plans of getting the D3D 10.1 back to Assassin's Creed, Ubisoft responded that there is no plan to re-implement support for DX10.1. Like we expected anything different from a TWIMTBP title.
You can check out the entire interview and some screenshots here