If I remember correctly, the Nexus S doesn't have hspa+
Only the G2 and MT4G have HSPA+ on T-mobile
you should still see 3-4mbps with the nexus S in good coverage areas, which i hear LA is.
HSDPA's theoretical limit is 7.2 mbps. In areas that have HSPA+ towers, even phones that have HSDPA have seen an increase in real-world speeds. I get 3-5 mbps down on my Nexus One regularly in HSPA+ covered areas of Portland. With a strong signal I can get 5-6 mbps.Wait, how does nexus S get 3-4 mbps without HSPA+?