According to Outpost Gallifrey the new Doctor is likely to be David Tennant, who is in another Russel T Davies production, "Casanova" on BBC3 -- and who's done several Big Finish Dr Who audios as well.
OG is saying a budgetary squeeze on top-shelf salaries may be part of the reason Chris "scarpered." (There's one of your other reasons, PHJ...)
Still developing...
http://gallifreyone.com/newstv.php