If you were able to backup the harddrive image, mod it with software, and then replace the harddrive image, you can still bring it in for warranty repairs. How would they know? But if you cut open the case, that is a whole different story.
They have one of those tamper seals over the case screws, so just by opening the case to your Xbox you are voiding your warranty.
The way I understand it is this: Softmodding might not void your warranty, but it might as well, because if your hardware ever breaks and needs repair, you're not going to be able to "put it back the way it was" before taking it into the shop. So, you take it in, they fix your Xbox, boot it, see a hacked dashboard, and say "you modded it so we're going to bill you for the repair."
