Time Machine does not back up to network drives, except Time Capsules or USB drives connected to an Airport Extreme, or a drive on a Leopard server.
Here's the official word:
http://support.apple.com/kb/HT1733
You may find some "hacks" that might make it work, in some cases, to a degree. But use them at your own risk.
First, since this is *unsupported by Apple,* there's nowhere to go when there's trouble.
Second, you're risking a future update preventing it from working, and perhaps rendering your backups useless.
If you still think this is a good idea, do yourself a favor before you bet your backups on it: be sure it really works, at least for now:
1. Get a copy of CarbonCopyCloner or SuperDuper! or the like, and make a full bootable clone of your internal HD, on an external disk. Test it by booting from it. Then reboot from your internal HD.
2. Let TM do it's first, full, initial backup.
3. Attempt a full system restore (like you'll need when, not if, your internal HD fails), via the instructions in item 14 of the +Frequently Asked Questions+ post at the top of this forum.
Again, even if it works now, tomorrow is another day . . .