Rule one... The Doctor lies. I do agree with you, though. He's usually quite an active interviewee, but in this case he doth protest too much, methinks. As far as I'm concerned it would be huge shock if Tennant didn't appear. I'd go so far as to say it would be ridiculous. So I think he knows we all 'know' he's going to be in it, and we know he knows we all 'know'. So his media appearances have become sport. He's asked, he says no - finding new ways to obfuscate the situation - and both parties laugh thus ending the contest. And repeat. I think he loves it.