Users of Sprint with 4G LTE devices in Washington DC, San Francisco, and New York City should be able to pick up spotty LTE coverage. It appears that Sprint has started flipping the switch on towers in those cities without making an official announcement. It's likely they're testing the towers ahead of an official launch.
Coverage has been spotty meaning that not all towers have been turned on. Once the coverage is officially announced and made live, users should expect a blanket of LTE goodness. Sprint has confirmed that the cell towers were turned on for testing and that the company decided to leave them on until the official announcement so that users could continue using LTE.