Which tool are you using to stitch panoramas? Every once in a while I can see a vertical line where they were stitched, but otherwise the results are outstanding.
I use the "stitch assist" mode on the camera, which keeps half of the previous picture in the viewfinder so I can line up the next one, and then I use the stitch utility that came with the (Canon s3) camera.
Usually works pretty good, BUT in Stitch Assist mode, it inexplicably takes the photos at 640 x 480! Might be a changeable default...