I second the use of rsync to do backups. A few notes:

- if you want to get rsync, you can do it by installing the program "grsync" (available in extras). However, if you don't know how to get rsync by command-line, you may not have enough expertise to create and run this shell script. But someone might want to create a GUI for the shell script and include the "apt-get install rsync" part or something.

- I do something similar, but I only back up my home directory, "/home/user/". I don't think we need to back everything up and then tell it to ignore the various system directories like "/proc/", etc. I figure I can always restore it to a known state.

- only backing up the home directory also obviates the need to connect and run as sudo, which makes it much safer.

- before I do a backup, I also back up my list of what packages I have installed. The shell script contains the command "dpkg --get-selections > /home/user/MyDocs/installed_pkgs". If your N900 gets wiped, you would restore using "dpkg --set-selections < /home/user/MyDocs/installed_pkgs"

- I also make use of the rsync options "-b --backup-dir=/path/to/backup/dir/", "--progress", "--partial" (you can replace "--progress --partial" with "-P") and of course as suggested by GameBoyRMH, "-v" so you can see what's happening rather than having rsync be silent for an hour leaving you wondering whether it's crashed.

- I use "rsync -e 'ssh'" to connect via SSH. In case you want to connect via rsync to a different port on your home Linux server (I think it's a very bad idea to use the default port of 22), you can say "rsync -e 'ssh -p 13579'", where you replace 13579 with whatever port you're using. Note that the entire 'ssh -p 13579' has to be quoted as the argument to "-e", because it is the ssh program itself that will be using port 13579.
I find myself unable to use shell variables to pass arguments containing spaces. So if I say
SSH_COMMAND="'ssh -p 13579'"
then if I say "rsync -e $SSH_COMMAND", it will say "I don't know what you mean by 'ssh", and then try to process "-p" and "13579'" separately.
Anyway, that's a minor point.

Thanks to rsync, I can make backups of my N900 while taking a walk in the park, and be reassured that the video I just took of my cute little baby will be backed up in case the N900 falls into the river before I get home.