Breakdown of how the iTunes Wi-Fi Music Store works

When an iPod touch is in range of a Starbucks T-Mobile hotspot, a fifth button appears on the iPod touch display.  So that requires the wireless driver to continuously poll for Starbucks' SSID, or possibly trigger a callback on detection of a new SSID which matches certain criteria.  Easy enough.

The clutch feature of the store client is that you can press a button and buy the song which is currently playing in the Starbucks.  Great for impulse buying.  But how does it work?  There are several possible explanations, but what really seemed to be the hard part would be purchasing a song right when it stops playing, or shortly after.  Those are the real impulse purchases.  How do you synchronize the user clicking 'buy this track', with what is currently playing?

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