Looks like the all the iPhones have the same radios. So, if an unlocked iPhone doesn't work on 3G with your current carrier (i.e. T-Mobile in the US), then you won't be able to find an iPhone anywhere that does.
From the Apple site (I verified with the top 4 cheapest countries):
Cheapest:
#1 Hong Kong
#2 Singapore
#3 Canada
#4 Switzerland
Looks like the all the iPhones have the same radios. So, if an unlocked iPhone doesn't work on 3G with your current carrier (i.e. T-Mobile in the US), then you won't be able to find an iPhone anywhere that does.
From the Apple site (I verified with the top 4 cheapest countries):
* 3G Support: UMTS/HSDPA/HSUPA (850, 900, 1900, 2100 MHz)
* 2G Support: GSM/EDGE (850, 900, 1800, 1900 MHz)