I went to a presentation on Di2 at my bike shop last week. The Shimano factory guys did the usual "wink and a nod" thing saying officially it only supported going up to 27, but that many people had gotten it to work with 28. It depends on the specifics of the frame in some way I didn't understand.

I bought it, getting it installed this week. I'm going to have them try to get it to work with my 28 cassette. If they can't I'll switch to a 12-27 and cry all the way up the hills, I guess.