I found out some more about the Xbox 360s DVD-player:
In the on-screen menu of the Panasonic PT-AE900 it actually says what input it gets. So when my Xbox 360 Display is set to 720p it puts out the following signals (I live in Belgium, Europe - we have PAL):
With the switch on the Xbox360's HD cable set to SD, you get:
And as stated above, the artefacts disappear. So they must be due to the Panny's internal scaler/de-interlacer not being able to handle 576p. I guess there's not so much you can do about that. Changing the Panny's settings (Cinema Reality, NR,...) doesn't help one bit.
Changing the Xbox360s Display settings to 480p or 1080i doesn't help either: it keeps putting out a 576p DVD-signal. Maybe I can sue M$ for that and get really rich and buy a nice HD-DVD player. I guess that won't happen though, cause I reckon it's the actual DVD disc that determines the signal. PAL = 576 lines any which way you turn it. I'll try and find me an NTSC (480 lines) DVD to prove my point here.
Maybe this is why we don't see any artefacts on DVDr as well.
One thing I still don't understand though is that the artefacts aren't on every (PAL) DVD I play: most of them have it, some of them don't.
Oh well, I guess I'll be using my crappy Sony-DVD player in the end: there's not that much difference with 576i.
If there's one thing I've learned from all this, it's that progressive vs interlaced really does make a BIG difference, especially when you blow your image up to a 100 inch diagonal.