When the Leica T was released, a unibody DX cropped sensor compact, it was quite a revelation, it showed Leica were into pushing technology forward, and while it had issues (slow and no built in viewfinder), as each new firmware was released it got better and better.
The Leica T with its T mount did interest people because the mount was huge, yet the sensor in the T was just DX, people hoped that this meant a full frame version was coming. Well that arrived with the Leica SL and the T mount was renamed the L mount, with the lens for the T re-branded TL.
The T has now become officially discontinued but this week saw the release of a new TL lens, a DX crop 60mm macro. I think its safe to say that the T system will continue for now in both DX and full frame forms, with a new T coming soon and the SL its mirrorsless big brother giving people the choice between 35mm crop DX and full frame autofocus cameras and lens and the equipment size and depth of field options this gives.