[CentOS] Boot message about "resume device"

When I boot CentOS 5.5, I receive the message:

Unable to access resume device ( UUID = some UUID etc. )

How do I find out what actual device to which this UUID refers ? It does 
not appear to be a block device since it does not show when I try 'blkid'.
To what does "resume device" refer ?

The boot succeeds but I would like to know what this messages means.