The Office Communications Server 2007 R2 documentation has been online for a while although really only the client content. The server content has been loaded this week. I will share that due to logistics around the site the R2 content exists in the Server Products and Technologies hierarchy for the current time and we expect it to be under Office System again soon. I also noticed today that I was not able to successfully navigate using the left hand menu navigation so please be patient as that is also worked out.
While there are those of us who have been working with the product for a some time due to beta use, most people will be seeing the product for the first time and that is why I have highlighted the migration content explicitly. In case the navigation is still a problem this is the link for the top of the R2 server content hierarchy allowing you to select content in the main (content) pane. http://technet.microsoft.com/en-us/library/dd250572(office.13).aspx