I am getting the same problem, but with PAL footage. In FCP 6 I used to export with “current settings” on a 16×9 sequence and I would get a 720×576 movie. I would hit command J and change this to 1024×576 so that it would work as widescreen in iDVD. Now when I export from FCP 7 I get a movie that displays in the correct aspect ratio, but when I drop it into iDVD it is anamorphic. Also when I change aspect using command J in quicktime, it no longer will force it to display correctly in iDVD. And also the information about the screen dimensions in quicktime is odd, just as the OP had said… but it’s not just the numbers… the actual movie looks like it’s in the wrong aspect sometimes.
Quite frustrating. Anyone having the same problem?