I thought of the paradox machine, too. No idea what it means - but there's definite connotations there, what with the Doctor's line about it being the "end of the world". But I don't know that it's the same End of the World as in S3 - I was thinking more along the lines of the two universes colliding. What did he say at the end of Doomsday, exactly, that he'd cause the universes to collapse if he and Rose spoke again, or something like that?

Anyway, that's what I'm thinking, along those lines. Not sure how much sense that makes.