The 37 states that are participating are the 37 states that already make commitments. The structure of the deal is that the U.S. commitments currently, under the agreement on government procurement, include 37 states. These were not extended to Canada, but Canada did not have commitments at the sub-federal level. So the 37 states were made available to all the members of the agreement on government with Canada, except Canada.
On March 11th, 2010. See this statement in context.