A BIOS mod most DEFINITELY WILL void your warranty. It's no different really to a solder/wire mod. Whether they'd be able to detect it or not if the card actually dies, who knows, but if it at all boots, ie: massive visual corruption or something -- well then you're SOL.
Quoted for fucking truth!!!
This voltage modification will definitely void your warranty, PLAIN AND SIMPLE.
This is definitely not considered a softmod, soft modifications usually mean windows based software manipulating GPU clocks, GPU DDR clocks, GPU voltage, in a windows operating system environment, meaning NOT PERMANENT.
With that being said, I'm going to try this voltage mod as soon as my aftermarket cooler arrives.
Good deal!