For centuries, vineyards in the Douro region of Portugal were primarily used for the production of Port. But with the market fairly stagnant for that fortified wine over the past decade or so, Douro vintners have taken the table-wine plunge. Port producers who were once adamant that they would never make a table wine have reversed course, and found success in both financial and qualitative terms. But that doesn't mean that Port has suffered in the process. Wine Spectator managing editor Kim Marcus explains.