The SDK will go live on the DevNet site when Premiere Pro becomes available for download.
There's no breaking changes made to force a recompile. However, there is some new behavior in CS6 that may require some changes to certain existing plug-ins to adhere to stricter standards. For example, the new feature to not stop playback while applying and adjusting effects requires that video filters properly initialize any parameter data on fsInit.
For more info on this specific feature, see: http://www.video2brain.com/en/videos-13069.htm
In general, developers will want grab the SDK documentation to read up on the changes, and then test their plug-ins in CS6 when it becomes available.