The word on the street is that Mitchell Johnson knows of his fate and it has come from the horse's mouth.

Mitchell Johnson is not enjoying the Ashes. The traditional rivalry has tended to derailed the bowler's rhythm every time it comes around. Following Ashes 2009, he received the ICC Cricketer of the Year Award but that was not because of his achievements in the Ashes but rather the work prior to it.
This time the Ashes may have given him only one chance. That is because after the first Test at the Gabba in Brisbane, Ashes 2010 will be a tough proposition for the Australian spearhead who is all set to be dropped for the second Test at the Adelaide Oval.
What has not worked in Johnson's favour is the fact that his own performance has been glaringly highlighted by the Houdini act that England pulled off and his own wickets column has been barren. The other thing that has worked against him is that Ryan Harris and Doug Bollinger have shown tremendous potential through their respective domestic cricket matches to make a strong claim against an opposition that Australia are beginning to fear losing face to.
For Ricky Ponting, that is even more relevant after losing two Ashes as the Australian captain. It may seem like a harsh decision but Australia do not want to take any chances, and if it means leaving aside a bowler who has made a tremendous impact in the last two years, it only reveals Australia's sense of urgency to find their feet against England.