Pietersen averages 46.83 post-Ashes
Bell averages 51.18 in the same period
Collingwood 48.47
Flintoff 30.37 and 28.04 with the ball
As I've said for quite sometime, I'd be quite happy with Flintoff at seven, with all three of KP, IB and PC playing. Flintoff, Hoggard, Harmison, Panesar is not an attack to be sniffed at, Collingwood, Bell and Pietersen can all turn their arm over if need be, though they do have only one Test wicket each.
It's not an issue of KP's performance not being good enough, it's the fact that the other two have performed better. KP averages 50+ against the Aussies though and still stands out as the most likely to succeed down under, especially against Warne and McGrath.
Throw in Cook (54.35) and if we bat down to Flintoff at 7, it's quite the middle order