This excerpt taken from the AAPL DEF 14A filed Jan 12, 2010.
How are broker non-votes and abstentions treated?
Broker non-votes and abstentions are counted for purposes of determining whether a quorum is present. Only FOR and AGAINST votes are counted for purposes of determining the votes received in connection with each proposal, and therefore broker non-votes and abstentions have no effect on the proposal relating to the election of directors. In the case of each of the other proposals, broker non-votes and abstentions have no effect on determining whether the affirmative vote constitutes a majority of the shares present or represented by proxy and voting at the Annual Meeting. Approval of these other proposals also requires the affirmative vote of a majority of the shares necessary to constitute a quorum, however, and therefore broker non-votes and abstentions could prevent the approval of these other proposals because they do not count as affirmative votes. In order to minimize the number of broker non-votes, the Company encourages you to provide voting instructions to the organization that holds your shares by carefully following the instructions provided in the Notice.