Having finally got Time Machine to work on my NAS following the other hints on this site, I came across a big problem. I set up Time Machine on my laptop, which is not always connected to my home network. When out on the road, it would try to sync and fail, which was good. However, when I am out and about, I also use my VPN to connect back to my house. This, I found, would allow Time Machine to back up. At first thought, that sounds good ... but doing so when connecting via 3G really doesn't work well.
So I changed the configuration of Time Machine such that it would not automatically backup, but rather do it when I tell it to do so. Go to System preferences » Time Machine, and make sure 'Show Time Machine status in the menu bar' is checked, then turn off Time Machine.
Now in the menu bar at the top of the screen, click the Time Machine icon and select Back Up Now when you want to back up. This finally allows me to back up when I'm on my home network, and not at other times unless I really want to. Of course, this also puts the onus back onto me to remember to run the backup frequently.
[robg adds: The interesting tidbit here (to me, at least) is that Time Machine will run even when it's been set to "off." I'm also interested to see if others have different methods of making sure a machine only backs up when desired -- are there solutions that still allow automatic backup when at home, and manual backup when not directly on the home network?]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20090122063202910