In the next series of Dr Who when the 12th doctor (Matt Smith) starts...his companion will be Karen Gillan ...
She has already appeared in Doctor Who, playing a soothsayer in The Fires Of Pompeii. This isn't unusually, incidentally: most recently, Freema Agyeman landed the role of Martha Jones, having first turned up in Army Of Ghosts as a different character.
Her character has not yet been named, and we just know that she'll be stepping into the TARDIS for the first episode under Matt Smith and Steven Moffat's reign on the show. She's thus far signed up, we understand, for the fifth season, which kicks off in the Spring of 2010.
Matt Smith won't actually be seen as Doctor Who until 2010. David Tennant will be finishing his specials off first, and then Smith will be in the role from season five in Spring of 2010. He has had no appearance or link with the show before.
Since Donna has the timelord mind that won't be all that she has. It has been confirmed that Donna will appear in the two part special at the end of the year. Maybe she will regenerate into Karen Gillan.
Just a thought.