Time machine backup on another mac in my network
You can set up your iMac as a fully functioning time machine server by purchasing the OSX Lion Server from the Mac App Store.

This provides you with an official method free from hacks to allow you to backup individual machines such as macbooks etc to a single target machine.  I backup my Macbook Air to my iMac in this way.  For extra safety, I occassionally clone off this backup volume in the iMac to an external disk and store it out of the house.