hi david
i can't reproduce this problem on my system (mac os 10.6.8, scribus 1.4.1, curve from inkscape 0.48.2 in standard svg).
but i observe a bit a strange behavour here as well: when i change the line endings from 'flat cap' to 'round cap' they aren't displayed correct. when i change to preview mode or back from preview, or when i move the path a little bit, the problem is gone – looks like a screen display issue only.
but i think, your case is more serious – maybe you should file a bug report. (i'll check, if the display problem is worth a report too…)
utnik