First of all, create a bootable USB-Stick (see different Dreamplug-Manuals for OS-Installation) with enough free space on it.
Then boot your Dreamplug from this USB-Stick by changing environment variables in Marvell Bootloader accordingly.
After your System from stick has started (I use Ubuntu 9.04 because it comes with more tools installed), mount the root filesystem of internal SD-Card.
root@ubuntu~: mkdir /mnt/dp_system root@ubuntu~: mount /dev/sda2 /mnt/dp_system root@ubuntu~: cd /mnt/dp_system
When you list this directory you should see the whole filesystem of your internal SD-card. After executing these commands you are located "inside" the filesystem, now simply use tar to pack and compress the whole filesystem (use * here) to a single file, I stored that at the /home of currently running Ubuntu on the USB-Stick.
ubuntu:/mnt/dp_system# tar -cvzf /home/dreamplug_debianSqueeze_Mono2.10.tar.gz *
After that, safely unmount the sda2 partition of internal SD-card and your snapshot of internal installed OS is successfully done. Quite easy right?