Some of my titles on the Amazon Manage My Kindle site now have a flag with a drop down explanation which says: "This title has complex layouts and has been optimized for reading on Kindle DX's larger screen, but can still be viewed on other devices."

It was four threads below yours

http://www.mobileread.com/forums/showthread.php?t=48132

I'm going to close this thread. It's interesting, though, that the tag is showing up on old content.