This excerpt taken from the AAPL DEF 14A filed Apr 16, 2007.
Shares Purchased. The number of shares of the Companys common stock a participant purchases during each offering period is determined by dividing the total amount of payroll deductions withheld from the participants paychecks during the offering period by the purchase price. In the event that the Purchase Plans annual cap limits the number of shares of Company common stock that may be issued and sold during any offering period, the number of shares that each participant purchases in that offering period will be reduced in proportion to the respective amounts which would otherwise have been purchasable by each such participant if enough shares had been available to enable all participants to purchase the full amount they elected. Any cash not applied to the purchase of fractional shares will be applied toward the purchase of shares in subsequent offerings.