It's a SDI modded Philips 963 SA (the very best SDI player out there). The problem also occurs when I play the test video through my HTPC.
I've just read up on the manual of the test DVD. It sais this:
This test video is almost ingeniously suited to identify the so called "phosphor lag" problem. This effect can be seen with all displays which are using phosphor to create the images: CRT projectors, CRT TVs and also plasma displays. The artifact is easy to see: Concentrate on the fast moving fonts. On the mentioned types of displays you'll usually see a green trail following the moving fonts. The blue phosphor is most often very fast while the red and green phosphors are noticably slower. Even when the electron shooting (UV with plasma) is already gone, these phosphor mixtures are still glowing - that's why the bright font is followed by a colored trail.
So my plasma shows exactly what the test disc claims it would show. If anyone is interested, you can PM me, then I can make the test video available for you.