I think it's fair to say that there will definitely be a higher-capacity version of this device, probably without the phone/wireless capabilties, and Apple now has at least six months before they need to worry about it. Even when it does debut, since the iPhone is the far more complex device, there will be far less R&D to worry about in debuting it.
As for partnering with a carrier, that's almost a necessity in the mobile phone business. Further, I don't know if Cingular is subsidizing this or not (as Apple's deal with Cingular is far from a standard manufacturer/carrier relationship), but if they are that would be necessary to keep the costs reasonable (this is priced in the same way as the early Palm Treo's and Blackberries were, and it's a much more sophisticated device).
Also, let's not forget that Apple has partnered with Cingular to also provide a better experience for things like the visual voicemail. This requires carrier-side integration to pull off successfully.