End of school year and everyone seems to want movies. Here are the latest tips. I had to actually make a DVD. Hadn't dont that in a long time. iDVD in iLife '11 is about the same, so Apple isn't spending much time on this. I'd forgotten all about chapter marks and so forth. Don't need them in the age of .mov and .mp4 files. In any case, got a multiplexer error and it looks like you need to do a bunch of things to get rid of it...
Macrumors Forums suggest deleting the plists and the caches. This seemed to help although I didn't see these Advanced encoding settings. This probably applies to the previous release of iDVD (version 7.0 in iLife '09 and not 7.1 in iLife '11).
as suggested by upinflames900:
deleted library/caches/com.apple.idvd (folder)
and as suggested by mattashton:
Advanced > delete Encoded Assets
Advanced > Encode in Background (was grayed out, so I didn't need to turn off)