iOS 4.1 will hit on Wednesday, September 8th. The US website still displays the non-committal "Coming Soon" message, so we'll have to keep our eyes peeled. We're going to go ahead and guess that all those shiny new iPods will come to retail that day too... but don't hold us to that one. Like we said, it's only a guess.
Update: And now the September 8 date is gone again. Thanks, Eren!