This excerpt taken from the AAPL DEF 14A filed Apr 16, 2007.
Withdrawal. A participant may withdraw from an offering under the Purchase Plan at any time without affecting his or her eligibility to participate in future offerings. However, once a participant withdraws from an offering, that participant may not subsequently participate in the same offering. A participant will automatically be withdrawn from an offering under the Purchase Plan upon a termination of employment and, in certain cases, following a leave of absence, a layoff, or a temporary period of ineligibility.