According to an article in the Toronto Star, November 16, 2004,

Airport bad news for Pickering tenants

the government is planning to announce the development of the Pickering Airport on the lands first expropriated in the 1970s. 

From one of the maps, it appears that the airport lands will be butting up against the communities of Cornell in Markham at the soutwest corner, the Town of Claremont in the northeast corner and the proposed/planned Seaton Development to the south.  The Town of Stouffville is not immediately adjacent to it in the northwest corner.

On the following link to the federal government's website, you can see maps of the area of planned development.

http://www.tc.gc.ca/OntarioRegion/pickering/maps.htm