ST-One
Vice Admiral
My memory is really foggy, but I vaguely remember hearing somewhere that TAS was recently (2006?) considered canon. I have a strong feeling though that this is a simple case of false recollection. Probably a manifestation of some article I read on startrek.com and the documentary on the TAS DVD.
In any case, I see the Robert April name name-dropped a lot on official merchandise. He is certainly regarded as official canon by not only the fans, but by those who have worked on the show (Sussman, Rossi, Okudas). I'd say he is about as canon as McCoy's daughter Joanna, which to me means that he fits the bare minimum of requirements needed to be considered canon. I, of course, don't have the final word.
McCoy's daughter was supposed to appear in Season 4 of TOS. The very fact that there hasn't been a season four makes her as 'un-canon' as April.
Don't forget that neither of any of the 'official' books or novels nor TAS is considered as canon, as far as the TV-shows and movies are concerned.
The shows and films can (and do) contradict TAS and any Trek-book at any time.
But that doesn't mean that the books and TAS cannot be used as resources.