i've heard the same thing as well. had the nexus s 4g on sprint and download speeds from 3g and wifi as well are weak. looking to grab the at&t white version, but not sure if its a model thing that google and samsung overlooked. there's a thread over at xda that suggests you can root and somehow recover lost download/upload speeds.
By the way, our local carrier here, Docomo Pacific (owned by NTT Docomo in Japan) began selling these with 2 year contract as a present on Mother's Day. They were also selling the white Galaxy Ace with the AT&T band too as a cheaper alternative.