| The impact is determined by your stock plan. Some plans have provisions on what happens to outstanding options or unvested restricted stock/RSUs upon an approved leave of absence. Being called to active duty is likely to qualify for an approved leave.
In addition, the Uniform Services Employment and Re-Employment Rights Act of 1994 (USERRA) protects employees who return to a company after a hiatus for military service or training. It focuses on ERISA benefits (e.g., pensions). Workers must be rehired either in their former jobs or in similar ones with the rank and pay that they would have earned if they had not been called away. They are also entitled to full credit in vesting and accrual of benefits in qualified retirement arrangements such as 401(k) plans. In light of USERRA and common practices under most stock plans, most experts would say that the call to active duty is not a termination under your stock plan and that your vesting should continue during your military leave.
Holders of ISOs in particular may be concerned about losing the beneficial tax treatment of ISOs through a disqualifying disposition caused by an absence of longer than 90 days. In its final regulations on ISOs (August 2, 2004), the IRS confirmed that your employment relationship, and therefore your ISO status, is not interrupted when you have rights under statutes such as USERRA. (Without this protection, absences longer than 90 days would cause a disqualifying disposition, i.e. the ISOs would become NQSOs.)
Because USERRA is a federal law, it applies only to federal military personnel. It does not protect reservists who are mobilized in response to state disasters (including those that result from terrorism). To bridge this gap, some states have enacted corresponding laws to cover their reservists and members of their national guards who are summoned to duty.
The US Department of Labor has more information on USERRA.