From a business perspective, the amazing thing about the ipod/iphone isnt the device, it's itunes. The device is simply the enticement and lock-in to the service, which is where the real revenue is made.
For Google, the Nexus One is simply a delivery mechanism for Google Voice and associated services.
The trouble with other smart phones (including the iPhone) is that they are locked to carriers. Why is that a problem? Imagine your cellular phone company being like your phone company or ISP: you pay a flat rate for a pipe. No plans or charges per call/text/whatever. Your modem isn't owned by them, and nor is your landline phone (the concept seems laughable now).
It's a dream for you, and a nightmare for your cellular provider, who is pushed away from the customer to become a wireless packet herder. So how does a software/service provider reach your dream? By providing you the means to untether from the carrier. Once you're unfettered, you're much more likely to focus on what matters: the software services, and not the proprietariness of the network's packages. Google Voice aggregates your phone numbers into a single voice service, so that, effectively, you no longer need a number, except as a legacy.
Just go big with the thought for a second: imagine if everyone had untethered phones with services like Google Voice. Knowing your carrier-provided phone number would be like knowing your computer's IP address: useful for diagnostic purposes only. In the age of avatars and 'screen names' isn't it quite remarkable that we still have phone numbers?