KitGuru confirms AMD's warranty policy does not cover damage caused by the dual-BIOS function on the Radeon HD 6990:
AMD confirmed with us this morning that the warranty will be invalid if the user switches to the ‘OC mode’ - which translates to a core increase to 1.175 volts and a clock speed of 880mhz.
We would strongly advise that if you are buying a HD6990 to leave it in the default position. Our testing shows that the cooler is doing a reasonably good job, but we can’t comment on the long term reliability right now. It is an unusual decision to offer an optional bios setting ‘out of the box’ and not deliver warranty support, but we would assume AMD might also have concerns with partnered power supply failures.