I have been asking this question to factory reps this past couple weeks, since you posted.
Basically the concenses is that a class "T" amp is nothing different than a class "d" amp, just somebody wanted to have there own letter. Class D amps are quite common, though not in headunits. There is nothing digital about class d or class t amps.
The problem with these amps is that they basically reproduce noice at a certain frequency that's very audible, has something to do with the reproduction of the sound wave, it was over my head when it was told to me. Very audible meaning around 1k or so. Thats why most class "d or t" amps are used for sub amps, because they can crossover below this frequency so it wont be heard.
There are companies who have figuired it out, like eclipse has a full range class d amp, but it sounds llke the money for the r & d and tooling isnt worth it, thats probably why you dont see them in headunits, manufacturing costs dont meet the demand or the justification of raising prices for a more effeciant amp.
It has also been the consensus that these amps, although super efficiant, are no where near the sound quality of class "ab" amps. This is why most companies have only incorporated them as sub amps. Some campanies have class d amps that produce very clean power and some that produce very dirty power, um audiobahn.
Alto mobile has been making these amps for probably ten years, some have said they were the first, but I doubt it, and they are pretty respectable for sound quality.
Hope that helps, Ive been meaning to pass this info along, you peaked my curiousity with your post, I will continue to ask people this question.