How do I prevent image scaling when burning an iPhoto Slideshow in iDVD?
The only way I've found to do it is to have iPhoto render the video, and then import the video file to iDVD rather than the slideshow directly through this "integration" of their apps and letting iDVD attempt to render it. 

This worked fine, and actually didn't take as long to re-render from .mp4 to MPEG2 as I expected it would. I experienced no- or negligible loss of quality to the video during transcoding, although I'm sure that there was some as I was going from lossy to lossy format.