I have a QNAP TS-209 at home, but I am not really happy with it. One of my concerns is security. With the firmware I've used (2.0.1 Build 0324T) there was a massive security hole.
The file .ssh/authorized_keys will be overwritten on ach reboot, with an unknown user (admin@Richard-TS209). See authorized_keys oerwritten at reboot.
I am not sure if QNAP has fixed this issue with the new Firmware, but this showed me, that my QNAP is not the device where I've want to store my private data.
Now it is possible on an easy way to install debian on your QNAP. See here is Martins description. Read carefully!
I want to show you here my experiences with Debian on my QNAP TS-209
The first step is to create a backup of the original Firmware
cat /dev/mtdblock1 > mtd1
cat /dev/mtdblock2 > mtd2
Save these both files to a USB-Stick or download them to your workstation!
Now download the required installer images
Run the following script to write the kernel and the initrd.gz to flash
This will take some time...
Writing debian-installer to flash... done.
Please reboot your QNAP device.