Nokia owns over 80% of the world market for cellphone Operating System (
Symbian) and User Interface (
S60), also licensed to LG, Lenovon, Panasonic, Samsung and Siemens. Apple and Nokia have worked together to put Safari and it's Web Kit engine on the latest S60 devices, N90, N80, N70 etc. Much of what makes iTunes work is the web engine, it wouldn't take much to port iTunes to the new S60 devices.
As a side note the N80 phone (should be out very soon) supports 802.11g. The cell companies don't like that and the Cingular version of the N80 may not have WiFi support. WiFi phones are a direct threat to the cell networks because they can't bill you for the time you use it. There are already VoIP clients for many S60 devices. Expect a war between companies like Skipe and Nokia and the cell network companies like Vodofone and Cingular. Apple may also be a factor in that battle. An IP based phone that uses a wireless IP network will have a small market to start with but it is the future of the cell phone. If the phone can also use CDMA and GSM networks it will have a very large market to start with.