check read permissions on the drive, fstab should have "users" in the second to last field
also check to see if you have libdvdread and libdvdcss installed
and, it might just be a dvd the drive doesnt want to read, I occasionally get a disc that my drive wont read, usually from the library
if you have mplayer installed it makes it a little easier to diagnose this, it has alot more output for debugging, not sure if xine has a log file or not
lastly, if xine will read it when run as root, its definitely a permissions problem, way to fix that is either make sure the drive is usable by all users, OR the quick and dirty way would be make xine run as setuid root with chmod u+s /usr/local/bin/xine