Hmmm,
jagged edges… this could be either interlacing artifacts or field dominance issues. When you go from Media 100 to iDVD you should do the following (at least for NTSC): MPEG2 used for DVDs is 720*480, while the “normal” video resolution e.g. for the Media 100 i codec or for ProRes is 720*486. So if you want to go to DVD you need to get rid of some lines. Unfortunately iDVD simply scales your video as a full frame without looking at the fields. Th avoid that you should export a DV-Stream from Media 100. Go to the File->Export menu and choose Other->DV Stream. Set the field dominance correctly (I think it is lower field for NTSC DV), Color range to “Computer” and in the options the video format to DV, Interlaced and NTSC. Since DV is as MPEG2 720*480 you will end up with a file with the right dimensions (and also the right colorspace) for iDVD. Burn that and watch your disc on a CRT. This way you can see if your fields are correct.
Regarding the jagged edges in web video: you need to deinterlace your video for distribution on the web. This depends on what program you are using to encode…