This Macintouch article was written for iPhoto export, but this is really a global issue with 16:9 vs. 6:4 vs. 4:3 video. The idea of "using a mask" in QT Pro to transform video is new to me, but sounds generally applicable.
iPhoto 5 added the ability to export slideshows with the Ken Burns effect included. However,iPhoto 5 exports slideshows to iDVD as 720 x 480 QuickTime movies. iDVD is expecting 640 x 480. When iDVD imports the 720 pixel wide movie, it squeezes the image and distorts it.
To quickly change the aspect ratio for the slideshow movie and have your pictures appear right, open the movie in QuickTime Pro and simply add a 640 x 480 mask to the video track and resave the movie. Now import the resized movie that conforms to the size iDVD is looking for.