So, I've got this pretty big (12 minutes, 3sec/slide) slide show. I may be done with it. I may not.
The show is composed of ten iPhoto-11 slideshows. Each one was set up 4:3, then exported, with the kind advice of Nathan in another question to specify H.264, 2048 bit rate, and the second-highest resolution (my pick on the theory that I was not going to be driving an HDTV, just a relatively ordinary projector).
Imported into iMovie-11 at 30fps, arranged, decorated with music, exported to via the share menu iDVD. Didn't touch anything in iDVD. just pushed the burn button.
Inserted resulting disk into nice new-ish dvd player with 30" LCD TV, 720 resolution. The pictures have a very visible structure of being built of little squares.
I dont' see artifacts in full-screen preview in iPhoto, or in full-screen imovie preview on my macbook pro. (On my mini, it warns me that full resolution will be degradated, so the presence of them there is not a surprise). But the DVD has this problem.
I discovered an option in iDVD to vary the 'encoding quality'. Moving that up to professional looked to me to make a notable improvement.