Sprint currently DOES offer 4G data service for its smartphones, but Sprint's "footprint" and signal strength seems to be best in and around the metropolitan areas it serves. It's network "shadows" Verizon's network -- but the actual signal strength is not as strong (at least in testing we have done) if you are slightly outside the main service areas.
Some other notes:
- Verizon is expected to offer the Iphone 4 in early February. Note that all of these numbers are confusing to some people. The Iphone 4 will used 3G speed for smartphone data service.
- Verizon's 4G service, called LTE, will be accessible by 4G phones that will probably be available in the second quarter of 2011. Verizon salespeople are saying, "Within a few months; maybe even in a few weeks" -- but it makes sense for Verizon to stagger their offerings.
- Sprint's 4G service seems good, but you should check the signal strength of your smartphone.