Today (in the US), most consumers purchase through retailers (wireless carrier stores and 3rd party retailers), and the phones are directly tied to specific carriers. The Nexus One was an attempt to marginalize this model by providing a direct channel together with a menu of carrier choices for the consumer to pick from. So far, the sales of the Nexus One show that direct to consumer hasn't worked. However, as the choice of carriers available for the Nexus One is just now more plentiful, its a bit premature to judge whether the al-la-carte carrier choice will have significant impact.
The problem I see for the Nexus One is that its now approaching middle-age for a mobile phone, and newer phones are certainly going to emerge in the coming months that steal the attention. So the al-la-carte carrier choice model may be too late in coming to have an impact, and therefore may not be the force for change in how consumers purchase phones and choose carriers.