You can use a network drive attached to another Leopard machine, though setting it up seemed a little random for me. I have an iMac and a MacBook, both running Leopard, and am now backing up the MacBook wirelessly to an external drive attached to the iMac, using Time Machine. Setting it up seemed a little random. I first did a backup to the external drive directly from the MacBook, then attached the external drive to the iMac. When I first brought up Time Machine preferences on the MacBook after that, it said it couldn't find the drive. If I picked the option to select a new drive it gave me nothing to choose from. Then I went and browsed the external disk in Finder, and suddenly noticed that the MacBook had now found the disk and was doing a backup (without me having selected anything new). There may be an easier way of setting it up, but the MacBook has been doing hourly backups across the network (which have generally been taking about ten minutes).