If the DVD firmware hacking is not working for your newly purchased Xbox 360, it might be because Microsoft has changed the version of the Hitachi drives they use.
So if you have a post-August 2006 manufactured console in the UK or Australia, and probably soon to the rest of the world, the firmware hack no longer works, at least for now.
By the way, if you want to know what version you have, there’s a helpful sticker on the drive.