Xine-ui requires xine-lib 1_beta*, while xine-dvdnav requires xine-lib-0.9*. This makes it rather impossible to use them both together, which is necessary when watching a DVD using Xine.
You are wrong. The libdvdnav code has been integrated into the xine-lib package since it moved to the 1.0 beta stage. When the time comes for xine-1.0, all the deprecated dvd playing plugins will be removed from portage.