I just want to say, that if Warne plays, Australia will definitely win the ashes. However, if I were Shane or Cricket Australia, I wouldn't want to play.
Australia have produced the best legspinner in history. However, they need to move on. In my opinion, if Warne plays for Australia at the age of 48 he will still excell.
But for now, they have to use Mcgill for a year or so before he retires. In that year, they can develop Casson while finding another young spinner in Australia (I don't know many) and develop him so that in the future so that for the next ten years Australia will be secured in the spin department.
If Warney comes back, it will mess up the reconstruction effort put in by the CA since the departure of some of their legends, and have been further hampered by Gilly's departure. Australia can't affort to look through the past for their answers - they need to try and create a bright future - who knows, Casson could develop into another Warne!
From a personal standpoint as well, I don't think that Australian could hope for a better end to test career than an Ashes series Whitewash at home - his record is untarnished, and if he comes back, everybad spell will be magnified, and even if he bowls well, which he will, his decision will always be known by the public as a mistake.