Sprint and Clearwire are the only wireless carriers offering 4G service that you can actually buy in America right now. The catch is that you have to be in one of the few areas that are covered by the service around the country.
Sprint and Lenovo have announced that they are teaming up to offer ThinkPad notebooks that come from the factory with 3G and 4G capability. The notebooks will have software that can connect to either 4G or 3G networks.
This is important considering 3G is still the most widely available mobile broadband and if users in 4G areas travel far from home, they will need 3G connectivity.