Thanks Nick, its useful to know that there are other ways of shunting data across the net, again this seems to be lacking encryption (not read everything yet).
You can use SSH tunnelling
for security, should do the trick.
I have to admit it's not the simplest solution - I remember fiddling with it a while back and getting into a mess with the combination of rsync authentication and Linux file permissions on the server (I did get it to work in the end, but there were a lot of bad words along the way). If you're a Linux guru, or have one handy, you should have no problems.
To be honest though, whatever you use, I think providing your own in-house remote backup service is fraught with potential problems, particularly as you'll presumably need to provide some kind of guarantee against data loss - you might be better off just signing your clients up to something like Mozy or Carbonite and leaving them to it.
Duplicati is incremental, only the changed parts of changed files get copied (it uses a slight tweak of Rsync so you only need access to a files on the other end, rather than needing an Rsync server)
I haven't come across this before, looks a lot better than Deltacopy if it does what it says on the tin.