It's been "upzoned" for years. Look at the city's zoning maps, there's a special TOD zone around Union Square in anticipation of the Green Line: http://www.somervillema.gov/sites/default/files/ZoningMapColor2010-03_0.pdf

(TOD = Transit Oriented Development)

To get the definitions of the abbreviations on the map read article 6 of the Somerville Zoning Ordinance: http://library.municode.com/index.aspx?clientId=14682&stateId=21&stateName=Massachusetts