Okay, I'll be the mad, insane, conspiracy-seeing guy in the room but... I'm sorry. This episode has not leaked.
There's one tumblr blog with shots of the box, along with a shot of the opening credits, in addition to the BBC tweeting about this. There are no companies named. There are no torrents out - and there would've been, no matter what ridiculous reasons the BBC make up to explain it away (there are reports that they're "shutting down torrents"). There are no other information.
The people claiming to have seen it? They're the usual fans, posting theories and making things up while selling into the story, lending their credibility either knowingly or unknowingly.
In addition, there's very, very rarely post on Sundays. Plus, the Tumblr-site (located here) reporting it appears to be the only one (which I suppose might be possible, but still highly unlikely) who's got it, and the only images coming from it. If this was a real leak, the internet would've been flooded with torrents and gifs. Even tumblr only has three or four posts claiming to have seen the episode, and they're saying things that don't necessarily have to be spoilers - they could easily just be theories.
And there's more; the Tumblr post went up about ten hours ago. BBC first posted about this maximum three hours ago, with Moffat saying that if the fans kept mum they'd release a clip of Matt & David. Now, do you seriously believe that for those eight hours, no torrents, no gifs, no clear, definite spoilers - not to mention a synopsis of the entire episode - would've leaked? I'm sorry but I'm not buying it. There are two solutions; either this is a complete hoax and a publicity stunt or there's one single person out there who has gotten his/her mail on Sunday and recieved the box by a mistake. If the latter is true, we'll probably see gifs, spoilers and synopsises, along with torrents, when postal services are up and about again on Monday. If it's a publicity stunt, which I'm fairly sure it is... Well.
That is one cruel stunt.
I don't have access to the Gallifrey Base (yet) so I can't check there, but there is no way in hell the BBC would've let a leak like this happen, and the fact that they've actually pulled something like this is... insensitive and cruel. They're going to make the fans tip-toe around the internet, ignore every rumor and post about Doctor Who so that they know as little as possible about the finale, then release the promised Matt & David clip when all the supposed good little fans have been nice and not spoiled anything.
I don't know whether to shake my head in disgust or give them some slow applause.