ATi (or CCC) OverDrive help doesn't state anything about warranty (it even doesn't use word "overclocking", but "manual change of GPU clock speed"). Every sentence of the instructions begins by words: "you can...". If the operating instructions nor the overderive help states anything about loss of warranty by using OD, than you can't lose the warranty - at least according to local law. Consumer is not obliged to check manufacturers website - all conditions and instructions must be included in fly sheet (paper or electronical) together with the product.