Personally I couldn't care if celebrities were or weren't involved in a game. It wouldn't make me run out and buy it. Sure it might be a nice bonus (Jonathan Ross did some funny lines in Halo 3) but to me and many others it means very little on the choice of purchase overall.
Nathan Fillion has a rather big role in Halo 3 ODST (He plays a Sergeant with looks like himself and voice's the character) and I think I remember somewhere that Jonathan Ross might be doing a few more lines.