Thanks for the thoughts. I'm starting to think that the issue lies in the Kindle 2's software itself. If I convert a bunch of images to .MOBI with the "no image processing" option in Calibre from large, high-quality images and also create a PDF on the computer with zero image processing (just combining the JPG's into a single PDF), they both look fantastic on the computer screen.
BUT, when loaded onto the kindle the .MOBI images look just as great as on the Mac (though they're limited to 622 height as per specs until zoomed manually), however the PDF images, while full screen (which is nice), look like garbage.
This leads me to believe that the Kindle's "experimental" PDF support just handles images in a nasty way. Perhaps the 2.5 software update that offers PDF zooming will also have cleaner image rendering/support?
Does anyone else know of/have experiences where the images embedded in PDFs were uglier than the same images displayed in the Kindle 2's CBZ/image viewer?