I'd doubt Carphone has ever send it to Nokia at all. Ask for the proof. If its really the case Nokia officially refused to repair, bring your evidence to the Nokia HQ US, CC any local customers unions along the way. Even in the some remote regions like Hong Kong every single employee in Nokia holds accountability for what they do and say, americans shouldn't expect less.

The longer you delay to take action the harder you can get the problem resolved. Worse still if you wait til the warrant expire.

In retrospective, you should contact Nokia directly for repair rather than rely on some unreliable agent. Nokia cannot wavie themselves of their reponsibilities of the defects of thier products, but some repair shop can.