Apple dont make any money out of the itune store. They make their money from ipods. Other download stores will find it very difficult to compete with apple because they're no sound business reason to sell downloads on their own. Hence their lacklustre "toe in the water" approach before Apple did it with "both feet first".
I think this is the first of the new technology wars that M$ has lost outright. Almost like a 5 day war, M$ had spent years trying to make wma succesful. Using there monopolistic ability (i.e. windows) to spead the gospel of wma. Overnight Apple have staged a military coup!!
All the third party mp3 players will adopt AAC very soon because its a free standard and there will demand it (i.e. windows store has already sold 1 million aac songs and it has 70% of the download market). M$ cant tell samsung/creative et al to not use AAC. Wma is gonna die pretty quickly and as big as microsoft is, they cant do anything about it. Who knows what the implications for DRM/WMA will be in longhorn if wma isnt succefull anymore..
Companies like M$, Dell, Apple etc.. are having to diversify to grow, the growth areas are not OS's, Wintel boxes or PowerMacs. The growth areas are consumer devices (ipod, xbox, cell phones etc..) and all these companies are re-positioning themselves. M$ has lost a major battle overnight and that is why they have been so public in their condemnation of itunes.
Also, Apples name on millions of PC's around the world is gonna do wonders for brand awareness. You can be sure Apple will be puting that to good use in the future ...