Lovely pair ... Billie and David
And Billie Piper, who plays the Doctors sidekick Rose Tyler, will also return for series three.
"There are lots of places for Rose to go," she said.
"I have had a ball ... I might still be here in 40 years."
David, 34, who last year became the tenth Doctor following Christopher Ecclestons surprise departure, appeared more circumspect about his contract, saying: "Mines not as simple as that."
But a spokesman for programme makers BBC Wales later confirmed David was "committed to Dr Who through 2007" for the third series.
The stars were speaking at the launch of the second series at the Wales Millennium Centre in Cardiff, which saw a screening of the first episode called New Earth.
The episode sees the Time Lord and Rose share a kiss, but the scene has a twist as Roses body has been taken over by Lady Cassandra, a survivor of the human race who featured in the previous series.
Asked if there were any nerves about the kissing scene, Billie joked, "we did not do tongues" and said they had been eating egg and cress sandwiches.
"Its not the last kiss of the series," David added.
Asked if the pair wanted to make a bid for success in Hollywood, Billie said: "Im loving whats going on here at the moment", while David said he had never had a "five-year plan".