If the boundary is still there when you have selected a drawing tool (ie the fill tool) then it is actually on the image itself. So either do as Jen said or you may have to rub it out by hand or use the magic wand.
If the boundary is NOT a border or the edge of a content box, but is just a visible edge of the top layer, it is generated in the human eye. To make that go away, the pixel colors at the edges of the top layer must match the pixel colors of the bottom layer where it meets the top layer.
did u opened blending option and checked STROKE? [it like double click on e layer and the BLENDING OPTION window will show up, the STROKE is listed bottom] if its checked unckeck it. [the boundary should be RED if Stroke is CHECKED]
or u can just select the Move button from Tools and click on the background.