I'm really worried about Jack being on a planet of Masters! I'm convinced he hasn't actually left Earth, but that's just me - where is he going to go?

Did you read my previous post? I talk a bit about Jack and his role in the Whoniverse. I'm pretty sure the reason the Doctor is/was squeamish about Jack (I'm still bitter about Utopia...) is that he really doesn't understand what he is, just that he needs him in the future/past (wibbly wobbly).