Totem gives an error message: The source seems encrypted, and can't be read. Are you trying to play an encrypted DVD without libdvdcss?
When run from the terminal this is the output:
libdvdnav: Using dvdnav version 1.1.7 from
http://xine.sf.net
libdvdread: Using libdvdcss version 1.2.9 for DVD access
libdvdread: Attempting to use device /dev/scd0 mounted on /media/cdrom0 for CSS authentication
libdvdnav: Can't read name block. Probably not a DVD-ROM device.
libdvdnav: Unable to find map file '/home/matt/.dvdnav/.map'
libdvdnav: DVD disk reports itself with Region mask 0x00f50000. Regions: 2 4
libdvdread: Attempting to retrieve all CSS keys
libdvdread: This can take a _long_ time, please be patient
libdvdread: Get key for /VIDEO_TS/VIDEO_TS.VOB at 0x00000130
libdvdread: Elapsed time 0
libdvdread: Get key for /VIDEO_TS/VTS_01_0.VOB at 0x000001d4
libdvdread: Elapsed time 0
libdvdread: Get key for /VIDEO_TS/VTS_01_1.VOB at 0x000051a3
libdvdread: Elapsed time 0
libdvdread: Get key for /VIDEO_TS/VTS_02_1.VOB at 0x00370696
libdvdread: Error cracking CSS key for /VIDEO_TS/VTS_02_1.VOB (0x00370696)!!
libdvdread: Elapsed time 0
libdvdread: Found 2 VTS's
libdvdread: Elapsed time 0
libdvdnav: Language 'en' not found, using '��' instead
libdvdnav: Menu Languages available: ��
AFD changed from -2 to -1
libdvdnav: Using dvdnav version 1.1.7 from
http://xine.sf.net
libdvdread: Using libdvdcss version 1.2.9 for DVD access
libdvdread: Attempting to use device /dev/scd0 mounted on /media/cdrom0 for CSS authentication
libdvdnav: Can't read name block. Probably not a DVD-ROM device.
libdvdnav: Unable to find map file '/home/matt/.dvdnav/.map'
libdvdnav: DVD disk reports itself with Region mask 0x00f50000. Regions: 2 4
libdvdread: Attempting to retrieve all CSS keys
libdvdread: This can take a _long_ time, please be patient
libdvdread: Get key for /VIDEO_TS/VIDEO_TS.VOB at 0x00000130
libdvdread: Elapsed time 0
libdvdread: Get key for /VIDEO_TS/VTS_01_0.VOB at 0x000001d4
libdvdread: Elapsed time 0
libdvdread: Get key for /VIDEO_TS/VTS_01_1.VOB at 0x000051a3
libdvdread: Elapsed time 0
libdvdread: Get key for /VIDEO_TS/VTS_02_1.VOB at 0x00370696
libdvdread: Error cracking CSS key for /VIDEO_TS/VTS_02_1.VOB (0x00370696)!!
libdvdread: Elapsed time 1
libdvdread: Found 2 VTS's
libdvdread: Elapsed time 1