This excerpt taken from the HPQ 10-Q filed Jun 8, 2007.
VIII. OPTION PRICE
The option price per share for the shares covered by each nonstatutory option shall be not less than 75% of the fair market value of a share of Common Stock of the Company on the date the option is granted. The option price per share for ISOs shall be not less than the fair market value on the option grant date. Such fair market value shall mean, unless the Committee deems otherwise, as of any date, the closing sales price for such Common Stock as of such date (or if no sales were reported on such date, the closing sales price on the last preceding day on which a sale was made) as reported in such source as the Committee deems reliable.