To hide a volume in Mac OS X 10.6 you will need to do the following
First, Install the Developer Tools (You can find these on the Mac OS X Installation Disc that came with your machine, or you can download Xcode from the Mac App Store).
Once you have done this Open a Terminal Window and type the following
sudo SetFile -a V /Volumes/VolumeName
replace VolumeName with the name of the volume you wish to hide
when the machine has rebooted you will notice that the volume no longer appears either on the desktop, or in a Finder window
to reveal a previously hidden volume just type the same command but use a lower case v
sudo SetFile -a v /Volumes/VolumeName
A common use for this would be to hide a volume with sensitive data on, although this will not encrypt any data so is not the best method for confidential items.