This excerpt taken from the WDC DEF 14A filed Dec 15, 2006.
Revoking Your Proxy
You have the power to revoke your proxy or voting instructions at any time before it is voted at the Annual Meeting. If you are a stockholder of record, you may revoke your proxy by submitting a written notice of revocation to our Secretary, by submitting a duly executed written proxy bearing a later date to change your vote, or by providing new voting instructions electronically via the Internet or by telephone. A proxy will not be voted if the stockholder of record who executed it is present at the Annual Meeting and votes the shares represented by the proxy in person at the Annual Meeting. For shares you hold beneficially in street name, you may change your vote by submitting new voting instructions to your broker, trustee or nominee, or, if you have obtained a legal proxy from your broker, trustee or nominee giving you the right to vote your shares, by attending the Annual Meeting and voting in person. Please note that attendance at the Annual Meeting will not by itself constitute revocation of a proxy.