Well, it's official: Catherine Tate is the new assistant for Doctor Who. To be honest, I do like Ms Tate and that I wasn't exactly hooping for joy when Billie Piper was originally cast (yet, I grew to like Rose and almost shed a tear when she left...) so I'm keeping an open-mind, but after the superb episode "Blink" many Whovians were calling on Carey Mulligan to be appointed as the new assistant, and I was one of them (not that I'd call myself a Whovian, more like a proto-Dr Who fan). She was not only very pleasing on the eye, but also gave a very good performance as Sally Sparrow, the episode's protagonist.
Also, I always thought the Doctor's assistants were younger than he was, which added to the whole "uncle" persona.
Anyway, I'm sure Russell T Davies knows what he's doing... I hope, though I reckon he should be giving more episodes to Steven Moffat and Paul Cornell.
Just my five-cents worth - excuse the Americanism.