Finally – official confirmation from the Doctor Who website!

John Simm will return as the Master in the two special episodes which bring David Tennant’s era as the Doctor to a stunning conclusion. There’s been speculation about Simm’s involvement with the show but we can confirm he’ll be back later this year, once again playing the Doctor’s deadly nemesis.
Simm first appeared as the Master in Utopia, The Sound of Drums and The Last of the Time Lords. All three episodes were written by Executive Producer and Head Writer Russell T Davies who also wrote his upcoming adventure.
The character debuted in 1971 and on-screen has faced all but the Second and Ninth Doctors. Davies’ version of the Master saw him retain his wit and cunning but since Utopia he’s been callous and charismatic, sick, psychotic and enormously likeable. In short, everyone’s favourite megalomaniac.
At the close of Series Three we saw the Master gunned down by his wife and whilst lying in his enemy’s arms, he refused to regenerate. The world believed he had died and the Doctor accepted he was the last of the Time Lords. But this is Doctor Who and you can’t keep a good bad guy down!
We’ve yet to find out how he survived but we can promise we’ll be bringing you an exclusive interview with John Simm and some brand new Master material later this year. Stay tuned!

Taken from io9′s summary of the Doctor Who Comic Con interview with David Tennant, Russell T Davies, Julie Gardner and Euros Lynn.

And then there was the trailer for Tennant’s final two-parter, which appears to be called “The End Of Time”:

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.

Russell T. Davies did confirm that Lucy Saxon, the Master’s long-suffering wife, will be back in the final episodes, alongside Simms. And the words “He will knock four times” have a great significance.

Taken from an interview with David @ this year’s San Diego Comic Con:

And Tennant admitted for the first time that season three’s villiain, John Simm really is coming back. And Timothy Dalton is also guest-starring, as we already pretty much knew. “Bloody hell, Timothy Dalton,” he says. “Such enthusiasm to be there.” “What a voice that man has,” says Gardner. And Wilf (Bernard Cribbins) plays a “huge and fundamental part” in Tennant’s final episodes, and brings “such humanity” to the role, says Tennant.

Great news!

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