Bell has been in terrific form. I feel that he could bat no5 at the moment. Colly is not so good at the moment and might be better at no6.
I'd probably resist the temptation of moving him up the order any further because his record does not suggest he does so well. That being said, I don't think he fears this Aus attack like the one last time in Oz. And the conditions in Eng 09 were much more bowler friendly.
I suspect that the only bowler he really has to be careful with is Johnson because of the left handers angles. His technique is so good that he has a very good chance against conventional right handers.