In the latest version of Ubuntu I noticed that partitions in fstab are referred using UUIDs instead of device names. If you want to find out the UUID for an existing partition then you can use a couple of rather obscure commands: blkid and vol_id.
blkid can be run as a regular user and without arguments will list all partitions and the corresponding UUIDs and labels. It can also be used to search for a partition if you know the UUID.
vol_id has to be run as root and you have to specify a partition.
UPDATE: You can also use tune2fs -l, as root.
UPDATE2: You can also use udevinfo as described in this blog post:
http://www.joeblog.info/index.php?/archives/369-guid.html
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