And the Doctor's new assistant is..
Freema Agyeman, who had a cameo (though playing a different character) in last Saturday's episode.
Much as Billie Piper brought credibility to Rose Tyler, and as good as her character is, I think it was the right time for a change.
That said, the new assistant doesn't appear until next season so the Doctor will be companionless for the Christmas special, although that has happened before in the series' history.
That said, I think the dynamics between Doctor and companions works best when he has two companions instead of one (depending on who the characters are), but then Jack Harkness may well be making a return next season!
Watch this space!