My understanding is the same as that of Ghengis, with the caveat that I gather the UK CAA have said that mandating it will be contingent upon development of a satisfactory technical solution - the so-called light weight, low cost, transponder. The specification is, I believe, for modes A and C with the possibility of adding elementary Mode S at a later date. The mock-up I saw years ago included a self-contained battery.
Trials with a prototype have taken place. I understand they were satisfactory.
I know of no commercial marketing plan yet. There may be a three-sided chicken-and-egg situation - the makers won't go commercial until the demand is there for volume production, that won't happen until it becomes mandatory, and the CAA won't mandate it unless it is available, if I believe everything I have heard.
Start saving your pennies now - and think of where you are going to put the thing, which might be on a strap round your neck if there is no room on the panel.
Chris N.