I think you skipped a step...you need to tell FCP to output 8 discreet channels. It's default it 2 outputs, you have to tell it 8 outputs. Here, I have a tutorial on the topic:
http://library.creativecow.net/ross_shane/multi-audio/1
As for iDVD...I dont' think it burns anything but a stereo mix. I don't think it'll do 8 discreet channels of audio.
Shane
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