From Sci-fi.com: David Tennant, told SCI FI Wire Freema Agyeman will join the show in the third season on the BBC as medical student Martha Jones. "The Doctor and Martha have a very different relationship to the Doctor and Rose," Tennant said. "The Doctor doesn't really think he needs a new traveling companion at the moment, but Martha makes herself indispensable."
Don't expect the Doctor to get over Rose so quickly. "As with any big relationship, it takes time for the scars to heal," Tennant said. "Perhaps the Doctor feels like he's dealt with it, but Martha might disagree." The second season of Doctor Who was released on DVD on Jan. 16. The third season will begin airing in Great Britain on the BBC this spring