Just a question, are the two sides of the face joined?

Make sure both sides are combined (edit polygons -> combine I think, maybe just "polygons -> combine")

Make sure the vertices are merged (edit polygons -> merge vertices -- be careful that the threshhold's not too high else you'll be merging anything within X-units of each other... which if high enough could reduce your entire model to a single point)

Type polySoftEdge -a 180 to set all edges soft... telling the renderer to smooth the faces the edge borders across that edge.