I've got a theory, well two actually. The first one is that they have indeed found a large number of lost episodes, but not the 108, but the films of episodes that they have still retained, but not in broadcast form, for instance the original colour versions of Ambassadors of death and/or Mind of Evil. I believe that the surviving episodes from the 60s are not the original films but video recordings that involved the film being played on a screen and then someone at the BBC pointing a video camera at it to record it for overseas sales purposes. Can anyone else confirm whether thats what happened with the B & W era episodes?
The second option is the reason all these people in the Dr Who production office have signed NDAs only to then spill the beans about missing episodes to all and sundry is a clever bit of misdirection by Moffat and co. In reality they've got big things to come for Nu Who and don't want the fans to find out before they should and so have us all chasing our tails hunting down missing episodes. Thats my two penneth anyway.