Actually it has not been definitively answered with regards to specifics of what is required. The versatility of e-tube technology is still somewhat confusing and I think those running Ultegra Di2 6770 groups are wanting to know whether they can simply upgrade derailleurs and cassette or is there more to it than that in order to run 11-speed shifting from what was originally a 10-speed Di2 group.
The original question was regaring shifters. Here's an answer, as posted on day one of the announcement:
"The 6870 and R785 groups are cross-compatible with both 10-speed 6770 components as well as the new Dura-Ace 9070 components, but [not] Dura-Ace 7870, as it does not use the E-Tube software. This means riders with 10-speed 6770 have the option to upgrade to 11-speed without having to replace their ST-6770 levers or brake calipers." - http://velonews.competitor.com/2013/07/ ... kes_292856
I understand the issue with the cableing, and that is something left to be researched, given that Shimano did upgrade the cables to make them lighter/simpler, but they also did that to the shifters as well.