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Well, that's because 3.1 is the firmware that the iPhone's run.
iPod Touches' run on 3.1.1 (And there wasn't a 3.1, I think.)
Calm down -.-"
 
Happened last year with the 2Gs. It's a special version of the software for the new ipod. Future updates will be identical to other iPhones/iPods.
 
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