Reuben at hiresit.co.uk worked out a great way of turning off Raspberry Pi Underscan/Overscan on the Debian Release.
Bear in mind the newer release of Debian Squeeze no longer needs these settings as its set up in the configuration menu during the installation however these instructions will still work.
Read on to see simple instructions on how to disable the under scan/overscan, link to Reubens webpage for the full post and the end of this post!
As he wrote in his blog post:
Here is how to turn off the underscan setting:
1. Open terminal and type:
sudo nano /boot/config.txt
2. Add the following line to the text file:
3. Save the file (Control+O)
4. Exit, reboot and enjoy full 1080p HD Pi
You can also use the file for adjusting the amount of overscan/underscan on each side of the screen. Negative numbers add black bars around the edge, the higher the negative number the smaller the black bar. Positive numbers stop the image spilling off the edge of the screen.
See the full post here with more instructions and details at hiresit.co.uk: source