The trailer for David Tennant's last episodes of Dr. Who, The End Of Time, have been seen. io9 describes it as this - Spoilers ahead:
We see the planet Earth from space, with the sun coming out behind it, and Timothy Dalton's voice purrs: "It is said that in the final days of the planet Earth, everyone had bad dreams." And then we see weird laughing faces, followed by David Tennant looking quite perturbed. We see glimpses of Wilf, Donna, Donna's mother, and an African man in a suit, all looking very serious. And there's a scary snake-faced creature. And then there's a figure in a dark hoodie lurking ominously. Dalton narrates: "He returns." We catch a glimpse of Ood Sigma, the Ood who told the Doctor his song was ending. And then the Doctor standing in the background, with the guy in a hoodie in the foreground. And then the hoodie comes off, and it's John Simm with his hair weirdly bleached! His voice comes: "My name is the Master." And then a title comes on the screen: "THE END OF TIME." And the Doctor walks forward with flames in the background, his face looking smoky and serious. And the title says "CHRISTMAS 2009." And the crowd goes wild.