If you're a loyal T-Mobile customer you get to pay a loyalty tax if you want a Nexus One compared to if you are not a T-Mobile customer.
Can we expect LTE phones to work across carriers? If so, I anticipate a demand for change in the US when someone brings their out-of-contract ATT phone over to Verizon and realize they are paying as much as someone that got a subsidy.
When the N1 came out, however, people were for some reason surprised about this. There's lots of news about it: http://www.google.com/search?q=t-mobile+%22nexus+one%22+%22n...
Google seems to think the former case deserves to pay $100 more for the phone.
Otherwise, you have people entering (comical) 20 year contracts, which would be (practically) unenforceable.