No one knows when a new one will come out. They haven't been around all that long. Personally wouldn't expect any new models until sometime next year.
Just looking at the past history, it comes down to the pricing of the memory they're able to get.
And no, if history is any indication, there would not be three models available. The 8GB would simply disappear. The 16GB would be at the current 8GB price and a 32GB model would be introduced at the current 16GB price point.
Give it six-eight months and I am sure a new revision of the iPod Touch and the iPhone will come out, each sporting more memory for the same price. As bobtomay says, it all comes down to the price of memory. The iPod Touch and the iPhone are in highly competitive market segments. Apple has to keep advancing these products to stay on top. We all benefit. Ain't free enterprise wonderful!
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