When I first started looking at projectors - DLP and LCDs, I never noticed motion artifacts. I then read an article by Tom Norton where he described problems with a certain projector and listed the chapters and scenes. I checked these out and sure enough I saw what he was taking about. I then bought a projector that handled these particular scenes quite well, but didn't realize that they were just the tip of the iceberg. Now I am extremely sensitive to motion artifacts and really can't enjoy a movie unless I can bypass the internal scaling of the projector. Don't get me wrong - I still see some problems even with a good HTPC, but the improvement is so dramatic that it is really nothing to complain about.
I asked my son to look for the rainbow artifact during Gladiator. He looked for it and did see it a couple of times. This was with the LT150 and my HTPC - lots of contrast and a very dark room. He said he would never of noticed it if I hadn't asked him to look for it and that it was so infrequent that it wasn't distracting - who knows though, perhaps I've ruined DLPs for him. I wish I had hooked up the DVD player and run an interlaced signal for him to watch.
I think we may be able to throw age into the brew. I am begining to think that the older you are the less sensitive you will be to this problem - generally speaking of course. No doubt this may be why I don't see it - so there might be one positive about getting older - look ma no rainbow! http://www.avsforum.com/ubb/smile.gif
(of course there will always be some old fogey who will claim he sees it even after the projector has been turned off!)