Now, the August rumour has been going around for a while, especially since the BBC is bound to have cast the new part by then (we mentioned this in June, in fact). In fact, if Moffat is planning to give the new Doctor a cameo in the 50th anniversary special (the timing works, it'd be a nice welcome for the new actor and the time-bending nature of such a story gag is right up Moffat's alley), then a proper announcement in August will prevent the dangers of camera-happy fans working out who the new Doctor is from who's on set. Which would be embarrassing. The cancellation of Doctor Who Confidential did upset a lot of fans (and also Moffat), and it is rather nice that we'll get one more episode of it to celebrate the anniversary year. But if the Confidential is being aired in August, then it won't be the usual behind-the-scenes episode filming for fear of spoilers (Confidentials were usually broadcast after the episode they were associated with, after all). It's more likely to be along the lines of the first Doctor Who Confidential, A New Dimension, which just talked about the prior history of the show. There's been enough material since 2005 to justify another such episode. That being said, mid-summer is an odd time for new Doctor Who material, so perhaps it'll air towards the end of the month if it'll be this soon.
As it's Steven Moffat we're discussing, Peter Davison has quite a good chance at being in the special, as per "Time Crash". Except for the matter of Davison saying he didn't think he'd be in it, but that was back in March. It is still curious that all those names are on the list, since the Radio Times previously reported that Colin Baker, Sylvester McCoy and Paul McGann were categorically not going to be in the special, but that was the same interview where Baker thought that they were filming the special in April, and that manifestly didn't happen. Tom Baker, meanwhile, has been fairly cagy on the subject and hasn't talked about it much since this time last year, when the tabloids were throwing around the idea, so maybe he is lined up for the special after all. He is a fairly obvious candidate for a cameo.