I can see the argument for having Katich at 3, given his more classical technique and less of a weakness early in his innings. However, Ponting has proven himself batting at 3, and shown the ability to deal with new ball conditions as well as play as a middle-order batsman, and he's also the best all-round batsman in the Australian team, which makes him best for the number 3 spot. Besides, it's not like he's done anything wrong there or Australia are in desperate need of Katich up the order for greater stability.
If Australia were to develop a serious propensity for batting collapses of course, putting someone like Katich at 3 would be a better idea than it is right now.
Regarding Clarke, his form so far in the Ashes tour has been exceedingly promising, and I expect him to have a fairly good tour. I'd expect a few lean tests and a big hundred somewhere, coming out the other side with a similar average to what he has now.