I'm not convinced Smith is a good enough bowler to be Test standard just yet, and his batting is hardly compelling enough reason to pick him. He needs a few more years in Shield to hone his craft, he needs to make the side on the strength of his bowling alone, and any decent batting we get from him is a bonus. Age of the middle to lower order, or a complete lack of any other spin options might force our hand with him and he will probably get a start inside that time frame. Hopefully that won't be a detriment to his development.
Clarke from all reports is fit and his back was not troubling him at all through the Gabba test, well that's the official party line anyway. So unlikley he will be rested at this point. That leaves North as the only likely batsmen in danger of losing his spot. I'd say the chances of that happening are slim given the selectors track record with team changes, although Chappell may have served notice with the Hauritz / Doherty decision that those days are drawing to close. If they do drop North then you would think Khawaja would be next cab off the rank given he was in the initial squad, but they could do worse than Ferguson on his home ground.
I really can't see them dropping Johnson, even though he deserves a wake up call. Both Hilfenhaus and Siddle out bowled him, so it would be unfair if either of them got shown the door. I think adding Harris and The Rug to the squad is just a none too subtle hint to Johnson. Basically perform this test or take a break and get your act together.
Anyway I'll be surprised if there are any changes to the side. I'll also be surprised if Johnson's form improves any, paving the way for Dougy to return in Perth.