This excerpt taken from the HPQ DEF 14A filed Jan 27, 2010.
Leave of Absence. For purposes of Incentive Stock Options, no leave of absence may exceed ninety (90) days, unless reemployment upon expiration of such leave is guaranteed by statute or contract. If reemployment upon expiration of a leave of absence approved by the Company or a Subsidiary is not so guaranteed, an Awardees employment with the Company shall be deemed terminated on the ninety-first (91st) day of such leave for Incentive Stock Option purposes and any Incentive Stock Option
granted to the Awardee shall cease to be treated as an Incentive Stock Option and shall terminate upon the expiration of the three month period following the date the employment relationship is deemed terminated.