After several weeks of speculation and early reports on the Internet today, later confirmed in a BBC press release leaked nearly a day before it was due to be officially presented, Freema Agyeman will join the cast of Doctor Who in its third season as new companion Martha Jones, sidekick to the current Doctor, David Tennant. Agyeman recently guest starred as Adeola, one of the Torchwood desk jockeys in "Army of Ghosts," but will return to the series for the completely different role (much in the same way first season guest star Eve Myles will be doing so in the spinoff series "Torchwood").
Agyeman will be seen in the first regular episode of the new season and not the Christmas special, as confirmed in the press release which quotes executive producer Russell T Davies as saying, "We've got another surprise in store for that." There are currently rumors of special guest actors in that episode, possibly debuting as early as the end of this year's season finale (but not yet confirmed).
The press release is below, which confirms Agyeman's status as a companion in the third series. The official Doctor Who website also has a news piece on this, and features a small gallery of images with Agyeman and Tennant.
Also reported, so far, at BBC News, the Daily Mail (which says that this is the Doctor's "first black assistant," forgetting about Mickey last year), the Independent, icWales, CBBC News, The Times, The Scotsman, The Guardian, ITV, Reuters. More news media reports soon.