The 700 Mhz band can already be used (in part at least) in Washington State. The State of Washington currently holds a license at 700 MHz and can begin operation immediately if they wish. The only limitations are that the State needs to protect Canadian broadcast operations (for now)
inside Canada. Not too hard to do since Washington can pick channels that are not shared with the Canadian TV stations.
There are also TV translators in the US that are secondary to the State license and will need to vacate. In some cases, the State
could work around these as well.
For local agencies, they need to wait until adoption of the 700 MHz 'plan'. This plan is ready for submission to the FCC for final approval. Approval will run 90 to 180 days or so. At that point, local agencies (like KC Metro Transit which just announced the bid winner for their new 700 MHz voice radio system or Pierce Transit which is still in the bid process) can deal with the few translators and start building systems.
For info on the plan, see
http://www.region43.org
The future is now!