This is just a guess, I'm not sure, but double check your UVs and make sure they go edge-to-edge. If they don't, you might be getting black pixels along the edge of the map.
Oh, also, unrelated side note: The new render-to-texture utility can be setup in such a way as to allow you to render out normalized sculpt maps at any resolution you like. I know that's been a popular request. Here:
http://www.inivis.com/forum/showthread.php?t=5583