This excerpt taken from the ZOOM 10-K filed Mar 12, 2009.
(k) Stock-Based Compensation
Compensation cost for awards granted after January 1, 2007 is generally recognized over the required service period based on the estimated fair value of the awards on their grant date. Fair value is determined using the Black Scholes option-pricing model. Compensation cost for unvested awards granted prior to January 1, 2007 is recognized using the modified-prospective method. Under this method, the same estimate of the grant date fair value and the same attribution method used to determine the pro forma disclosures under SFAS No. 123 (R) , Accounting for Stock Based Compensation are used to recognize compensation cost.