According to http://developers.videolan.org/libdvdcss/, libdvdcss (which is used for reading and decoding DVDs by most Linux software) does not require the region to be set, even on the RPC2 drives described below. It manages to read from the DVD regardless.
Many thanks to Rob Teng for bringing this point to my attention. With this in mind it should not be necessary to set the region on the drive using the procedure below.
If you have a new DVD drive it may well be RPC2 which means the drive itself is supposed to check the region code on the DVD. One fix for this involves finding a cracked firmware for the drive that removes this check. However before doing this you might want to check your drive works for DVD playback before invalidating the warranty .. and unless you want to boot into Windows this requires a Linux program to set the region.
IMPORTANT: most drives only let you set/change the region 5 times. This limit is built into the drive firmware so you don't get round it just by running Linux! Please also read the note above about libdvdcss, you may find that you do not need to set the region on the drive to play DVDs under Linux!
dvd_region tells you how many changes are left (NB when changing the region it reports this incorrectly, don't panic!). You may need a DVD in the drive for it to work. Just run to see current status, use -s -r x to set region (where x is the region, eg 2 for Europe).
This manual page was written by Daniel Baumann <daniel@debian.org>, for the Debian project (but may be used by others).