Update 18 December 2007:
I’ve discovered a better way to force Finder to browse the contents of a network drive. You can read more here.
By default, Time Machine will not allow you to back up to a network drive – even if you can get Leopard to recognise the drive.
However, there is a command that will allow you to use a network drive with Time Machine. Before running this command, mount your network drive. Open Terminal, and at the prompt type:
defaults write com.apple.systempreferences TMShowUnsupportedNetworkVolumes 1
Open Time Machine Preferences, and you should now be able to view your network drive mounts.