CDMA
I think the users per mhz in the US is probably taking density into account. I think a big reason for it is CDMA. UMTS can really squeeze the callers in pretty well, but compared to (2G) GSM, CDMA is far more efficient and there's been nationwide CDMA deployments here for at least 10 years.
That said, the cell cos are mad. 800mhz more spectrum!? They have not even deployed the spectrum they already have! (1700/2100mhz, some at 2500mhz, the new 700mhz spectrum -- all purchased, but almost all idle.)