I would ask on
http://plugcomputer.org , which is the best program that can be installed on the sheevaplug to share files over the internet?.
I remember when it was last asked there is some good program which was put forward to do it.
Even Webmin as a built it file upload/downloader, so don`t think there is a problem finding one
I just use Secure FTP and samba and apache for a web server.
If you have a static IP address you could use FTP and map your computer to it.
especially when copying folders with large numbers of files, or backing up.
So you want to backup over the internet? , if so don`t forget you are limited to the upstream of your adsl , which in most cases isn`t that fast.