I solve this problem by keeping an archive of and stripping the DRM from every eBook I purchase. My weapon of choice is Calibre with the Kindle PID extensions.
My Kindle is still content to read all of the titles I've stripped, although usually I stick to the DRM'd versions on-device to allow Whispersync between that and my iPhone.
My Kindle is still content to read all of the titles I've stripped, although usually I stick to the DRM'd versions on-device to allow Whispersync between that and my iPhone.