Walter,
I spoke with the author of the post that indicated broadcasts began May 9, and he told me that was written in anticipation of that broadcast start date, even though the publication date was a week or so after May 9. Turns out, the FAA missed that broadcast start date, and has also missed the June broadcast start date as well. So much for everything on the Internet being true!
The author is also a member of the RTCA working group responsible for defining the content of the ADSB-In stream (SC-206 SG5). I asked him where I could get the details of how the new product data will be included in the stream, and he suggested RTCA MOPS DO-358A and B. I looked in their online store, though, and I only see DO-358 (with a pub date of 3/24/2015), so I struck out there, too.