The point in question was awarded to Stosur due to the fact that Williams let out a loud "Come on!" shout before the point was over. After all was said and done, I've seen some mentions on Twitter that the rule regarding this situation is different between the ITF (the governing body in Grand Slams) and WTA (who run the regular women's tournaments). Two related incidents were brought as an example - one in which the verdict was the same (in Marion Bartoli's match against Cristina McHale in this US Open), and an older one, involving both Williams and Asderaki, in which the point was replayed.
I decided to check the matter - and it seems that the "Hindrance Rule" is in fact very much alike in both sets of rules.
Here is the ITF version, which was relevant to the final match:
26. HINDRANCEThis is the rule in the WTA rulebook:
If a player is hindered in playing the point by a deliberate act of the opponent(s), the player shall win the point.
However, the point shall be replayed if a player is hindered in playing the point by either an unintentional act of the opponent(s), or something outside the player’s own control (not including a permanent fixture).
H. HINDRANCE RULEAccording to both those quotes, any deliberate hindrance will cause the player to lose the point, while any involuntary (note: this is not the same as "unintentional") hindrance will result in a let call and a replay of the point.
If a player hinders her opponent, it can be ruled as either involuntary or deliberate.
1. Involuntary Hindrance
A let should be called the first time a player has created an involuntary hindrance (e.g., ball falling out of pocket, hat falling off, etc.), and the player should be told that any such hindrance thereafter will be ruled deliberate.
2. Deliberate Hindrance
Any hindrance caused by a player that is ruled deliberate will result in the loss of a point.
In any case, Asderaki's decision yesterday was correct. While I'm sure that Williams didn't intend to disrupt the point, the shout could not be considered "involuntary", no matter how you look at it.