The two big cell radio technologies in the US are CDMA (Verizon, Sprint) and GSM (pretty much recognized by a SIM card: t-mo, att)... Both have different 3G technologies, but still call it 3G because they were created pretty much around the same time and have similar speeds.
4G is on the horizon, with some companies already having their towers rolled out in some test cities. Two leading methods for 4G are LTE and WiMAX.
Sprint is the only one going with WiMAX, with the rest going for LTE.
LTE is a lottttt faster than 3G and can also handle voice and data at the same time.
Also, because LTE uses internet protocol (IP) where both voice and data are treated as packets you will be able to do both voice and data at the same time.
Verizon should start rolling this out by the second half of 2010 and have it completely implemented by about 2013.
"AT&T Mobility has stated that they intend on upgrading to LTE as their 4G technology in 2011, but will introduce HSUPA and HSPA+ as bridge standards."
"Verizon Wireless completed its first test LTE data calls in August 2009 and plans to deploy LTE beginning in 2010 with system-wide deployment completed in 2013."
Oh and last time I checked a few months back, it was: start rolling out 2009 and done in 2012 for Verizon.... Looks like things are getting pushed back