"Tennant, who has played the main character of the show for the past
three seasons, stated on The Andrew Marr Show in June 2008 that he
hasn't yet been asked to reprise the role for the fifth series, but
he will consider it if asked." - From Wikipedia. Source:

http://www.gallifreyone.com/news.php#newsitemEkEkAFuEFlpkcjXcoK