Can I ask, is there really an animosity between Smith and Moffat, as its been lead to be talked about, thus leaving earlier than he would've? I don't really buy into that, but since you know of the mail and stuff, I thought I'd ask you.
From all available evidence there's no animosity between the two. They were photographed together in New York in May after reports surfaced that they had had a falling out. And Moffat is nothing but complementary when speaking of Matt Smith, his work, and his dedication.
Matt Smith did three seasons, plus a little more. Moffat has said Smith stayed longer than he (Moffat) thought he would. I wouldn't be surprised if Moffat made an effort to convince Smith to stay; we know that Moffat discussed series 8 with him, there may have been other enticements offered (such as writing and/or directing an episode, which were rumored), and Smith countered with this "two Doctors" plan (though some think that it was never a serious offer) to stay. If Moffat drove Smith from the series, I don't see Smith pitching him an arc and format for series 8, I just don't.
Though, for all I know, we'll find out five years from now that Moffat and Smith despised each other, just as it's come out in the last few years how badly Eccleston got on with RTD and Phil Collinson.
The thing to watch for, I guess, is how active Matt Smith is in promoting "The Day of the Doctor" and the Christmas Special.