To me it would make no sense to create an ipod with 3g option...why not just make the iPhone 4S available with a data only plan?
Because an iPhone 4S is probably going to be priced around unlocked iPhone prices now. If so, that option is already available to the public but some of that public doesn't want to pay that much for it when there are Android and Mifi devices that cover this need for so much less. The "sense" is in choosing to compete for THAT business rather than letting Android and Mifi companies have it.
Who's that market? The parents of teens/tweens who have kids begging for 3G-capable devices for mostly texting purposes. Those teens/tweens would love to be given an iPhone but the parents don't want to buy a $600+ device and a 3G plan. This iPod with 3G data concept (if the 3G piece matches the 3G options of an iPad 3G) costs about half for the hardware and only $15 or $25/month for them to be able to text like crazy.
As a parent of one of these teens myself, I'd buy that iPod 3G in a second. Last year, we did this same thing by deciding against buying an iPhone (because the total cost of ownership is too high for what will be mostly used as a texting device). So we bought our kid an iPod Touch and a Mifi device (because we could get a much cheaper 3G plan via Mifi than the minimum requirements with an iPhone). That's worked well for these purposes except she- and many of her friends who's parents did the same- would really like the Mifi part built into the iDevice (one device instead of two). We'll happily pay up for a new iPod 3G this year to give her that, but we won't pay up considerably more for an iPhone... unless an iPhone is launched unlocked (no 24-month contract required) at iPod-like prices (but then I think that iPhone probably kills the iTouch line- I mean, if you can get a "whole" iPhone for an iPod Touch price and it is not bound to a 24-month contract, why buy an iPod Touch?)
Why should Apple be interested in THAT business? Because these teens will accumulate apps on whatever device they are given. Then, in a few years, when they can buy their own "real phone" they'll be more likely to buy one compatible with the app library they already own rather than switching and having to re-buy the same apps again. Thus, for Apple, it's a strategic move aimed at locking in a share of first-time phone buyers looking to buy a new iPhone 7 or 8 in a few years.