The end of TNOTD pretty much answered the question. He's not a "forgotten" incarnation as the original speculation went since Smith knows exactly who he is. He wasn't worthy of being called "Doctor" which allows him to be one of his past selves which not dirsupting the numbering. (Matt Smith is the 11th Doctor but the 12th body for this particular Time Lord.)
I just hope the Moffat isn't going to have him be the one who ended the Time War as that would contadict Eccleston's entire arc.