According to Apple Support, the information that FCP 'speaks' whenever a 4:3 movie is actually an anamorphic 16:9 is not the same as that which Quicktime and iDVD will recognise. What is required is a quick couple of clicks in Quicktime Pro (bundled with FCP and FCP Studio products) to translate, as such.
Choose Window > Show Movie Properties.
In the Properties window, click Video Track in the Name column.
Click the Visual Settings button.
Deselect the checkbox for Preserve Aspect Ratio.
Change the Scaled Size:
For NTSC, enter 853 x 480.
For PAL, enter 1024 x 576.
Now the movie should be displayed at a 16:9 aspect ratio.
Choose File > Save to save the movie.
And you're done.
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