Aren't like... the radio bands slightly different for Verizon and Sprint phones? Meaning... you know... it wouldn't work quite right?
That's what I'm assuming. Kind of like how the OG Nexus that was on Tmobile was usable on AT&T, but you only had EDGE. I'm guessing you'd be able to USE the Nexus S on Verizon, but you'd only have 1x speeds. Not 3g, let alone 4g. Sprint is Wi-Max, Verizon being LTE.