ERIC DEL SESTO  This works well, in fact for some reason it works better than the manual approach (which is to enter 'defaults write com.apple.systempreferences TMShowUnsupportedNetworkVolumes 1' in Terminal). On a brand new iMac with OSX 10.5.5 I tried the manual approach, and it did not work. I used 'defaults read com.apple.systempreferences' to confirm that the 'defaults write' command had worked, but Time Machine was still not showing my external (AFS mounted) volume, even after a reboot. I upgraded to OSX 10.5.6 and still the manual approach was not working. I then downloaded and ran this utility and now Time Machine shows that external volume. I can't explain why this utility works better than the manual approach; perhaps it is doing something to cause OSX to re-read preference flags. Thanks, Antonello for creating this. Saved my evening! - Eric  
(Version 1.0b)

NALTS  I have not tried TM Manager, but wanted to give a warning to those that enable TM for use over a network.

First off, here's how to turn a networked disk on using the Terminal:

defaults write com.apple.systempreferences TMShowUnsupportedNetworkVolumes 1

Now that you know how, go read this post from someone who lost some data / backup while using TM. Makes me want to go out an buy an external hard drive and not use my NAS.

http://discussions.apple.com/thread.jspa?messageID=5918065#5918065

Thought you might want the heads up.  
(Version 1.0b)

DANA SUTTON  I've been using TM Managber for about a month to allow Time Machine backups to a shared server on a LAN, it works fine and has caused absolutely no problem for anybody on my network (all both of us). I understand that at the last moment before Leopard's release Apple disabled wireless sharing for TM because of some security issue, and evidently they disabled the ability to backup over a network at the same time. Probably there is a hidden option to turn it back on that can accomplished by a Terminal command, but nobody seems to know what that command is, so TM Manager is currently the only way to go. Highly recommended for those users who have the need.  
(Version 1.0b)