Time machine is a new feature in Leopard that backs up your data but by default it only supports backing up to another physically attached hard drive. There is however a work around to get it to backup to network shares;
- Go into Applications / Utilities and open terminal
- Type in defaults write com.apple.systempreferences TMShowUnsupportedNetworkVolumes 1 and then press enter
- Mount the network share (cmd + k)
- Go to System Preferences, open Time Machine and select change disk
- Then select the share that you want to backup to and Time Machine should then backup to it