If you want smartphones, AT&T is pretty much the best choice. Import the quad band 3g phone of your choice and you will be good to go. T-Mobile is 2nd, but no 3g, and when they do go 3g (maybe next year), they will use the 1700 mhz band, which no one else anywhere in the world uses. Time will only tell if manufactures like HTC will impliment 1700 mhz into it entire GSM lineup or only on the models they build fro t-mo like the dash and the wing (which, btw, neiter of them will support 1700mhz).
For CDMA, we may just beggining to have the flexibility GSM has soon. Sprint just setteted a class action suit and they will now activate any CDMA phone compatable with their system (Though they will not support any non-Sprint phone). Before, they would only activate a phone from their ESN database (which they sold, in effect). If Verizon adapts this policy (The next two biggest CDMA carriers, Alltel and US Cellular dont have the ESN restrictions), with CDMA growing in areas like China and India, and CDMA firmly rooted in S. Korea, we could see imports if they support the 800, 1900, and possibly soon to be 700 mhz bands.
edit: looks like S. Korean phones are pretty much out for CDMA. Most of the carriers over there are now using WCDMA (UTMS-2100 mhz) instead of CDMA2000 (EV-DO) which CDMA carriers use over here for 3g.