Well, google it.
I can only crush you dreams. I had an iMac with a few drives connected. I couldn't use any of the drives (they were shared) for a time machine of my macbook pro over wifi/wired network.
One time I did manage, but I think it was because I installed Snow Leopard Server on it. So you could look into that, especially if you want to make a back-up when you're not in your home network (e.g. at work).