I just spent the better part of the day (6+ hours) playing every variable I could think of to get Digital Editions to display my cover when saved from Calibre. Different sizes, converting from epub, converting from mobi (where the cover always showed), and so forth. I downloaded and installed the plugin Cover Generator, which worked just fine, and confused me even more.
And then I just read this: "the file was CMYK." OMG. I immediately checked and sure enough, the cover was CMYK, I resaved it from Photoshop as RGB, and redid everything in Calibre, and it worked the first time! Cover looks great in both epub and mobi formats!
Thank you for posting your solution!
Flying Chipmunk Publishing