Apple's limitation on what kind of drives (directly attached drives) can be used as a Leopard Time Machine backup disk can be overcome with a small command line utility.
All it does is make Leopard show unsupported drives as a supported drive for Time Machine, but Apple most likely removed support for these drives for reasons such as data loss or data corruption, so be careful if you're going to try this. Just head on over to 9to5Mac and download their aptly named Flux Capacitor automator script and you'll be on your way to network backups in no time. Or, you can just wait for Apple's official release that's sure to come soon. [9to5Mac via MacRumors]