Aside from the issues of doing deals with mobile network providers, it seems to me there is something of a gulf between the abilities of mobile networks and the core functions of iTunes and iPods.
Downloading music on even a 3G network is slow (and expensive with current per-megabite 3G pricing structures in the UK). On the other hand, boadband and Wi-Fi are natural partners for this sort of activity. As the availability of public wi-fi networks becomes inceasingly ubiquitous, the viability of a wi-fi enabled iPod / iPhone seems to me the most likely solution.
Vonage have already released their mobile wi-fi phone handset - ( http://www.vonage.co.uk/device.php?type=F1000 ). Doesn't really appeal as I would need to carry it around as well as my normal mobile for areas with no wi-fi. However, if such a device was also able to connect and purchase directly from the iTunes store, looked and felt like an iPod, and carried and played all my tunes and videos and also enabled wireless sync with my Mac, then I would certainly carry it around with me everywhere in addition to my traditional cell phone, just as I do my iPod today. Where there is (free or cheap) wi-fi available, I would use it for outgoing calls, elswhere I would rely on my mobile.
iPhone: iThink Not