Cross-border tensions or not, there's growing talk in Washington that Canada and the U.S. could expand their military co-operation in a continental missile shield. It came up last week at a U.S. Senate committee hearing, when the top-ranking Democrat reported he'd just gotten positive signals from Defence Minister Bill Blair that Canada could end decades of reluctance to join U.S. missile defence. Rhode Island's Jack Reed was talking about U.S. plans for a multi-faceted upgrade to the system — the project is currently nicknamed Iron Dome, a reference to Israel's existing, albeit vastly different, system. "I met recently with the…