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I constantly go back an forth between my iPhone and nexus one and never had any issues. I have an iPhone unlimited data plan with at &t
 
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Probably not. I upgraded my wife's 3GS to an IPhone 4 and since she was already in an unlimited plan, she was able to keep it. Unfortunately for me though? I terminated my contract a few months ago so now I'm stuck with a 2GB plan.
 
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