Don't forget, the original ipod was Mac only for a while, and then they eventually released a windows version when they saw how popular it was.
Apple always does this, because there major aim is to convert people into changing platforms by showing them that there are "must have" technologies only available on the mac (ie-they themselves indicated that the windows ipod was an inferior product to the mac version)
Since the lure of this new music service is likely to be its ease of use, there may be issues with implementing the same experience on windows, and that could prevent the eventual rollout of a windows version.
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