The valuation of stock options is a significant accounting estimate that requires us to use judgments and assumptions that are likely to have a material impact on our financial statements. Annually, we make predictive assumptions regarding future stock price volatility, employee exercise behavior, dividend yield, and the forfeiture rate.

The estimated fair values of stock options granted and the assumptions used for the Black-Scholes option-pricing model were as follows:

NOTE 11. STOCK PLANS

We use broad-based stock plans to help ensure that management's interests are aligned with those of our stockholders. As of May 29, 2011, a total of 16,942,290 shares were available for grant in the form of stock options, restricted shares, restricted stock units, and shares of common stock under the 2009 Stock Compensation Plan (2009 Plan) and the 2006 Compensation Plan for Non-Employee Directors (2006 Director Plan). The 2009 Plan also provides for the issuance of cash-settled share-based units. Stock-based awards now outstanding include some granted under the 1995, 1996, 1998 (senior management), 1998 (employee), 2001, 2003, 2005, and 2007 stock plans and the Executive Incentive Plan (EIP), under which no further awards may be granted. The stock plans provide for full vesting of options, restricted shares, restricted stock units, and cash-settled share-based units upon completion of specified service periods or in certain circumstances, following a change of control.

Stock Options

The estimated fair values of stock options granted and the assumptions used for the Black-Scholes option-pricing model were as follows:

We estimate the fair value of each option on the grant date using a Black-Scholes option-pricing model, which requires us to make predictive assumptions regarding future stock price volatility, employee exercise behavior, and dividend yield. We estimate our future stock price volatility using the historical volatility over the expected term of the option, excluding time periods of volatility we believe a marketplace participant would exclude in estimating our stock price volatility. We also have considered, but did not use, implied volatility in our estimate, because trading activity in options on our stock, especially those with tenors of greater than 6 months, is insufficient to provide a reliable measure of expected volatility.

Our expected term represents the period of time that options granted are expected to be outstanding based on historical data to estimate option exercises and employee terminations within the valuation model. Separate groups of employees have similar historical exercise behavior and therefore were aggregated into a single pool for valuation purposes. The weighted-average expected term for all employee groups is presented in the table above. The risk-free interest rate for periods during the expected term of the options is based on the U.S. Treasury zero-coupon yield curve in effect at the time of grant.

Any corporate income tax benefit realized upon exercise or vesting of an award in excess of that previously recognized in earnings (referred to as a windfall tax benefit) is presented in the Consolidated Statements of Cash Flows as a financing cash flow.

Realized windfall tax benefits are credited to additional paid-in capital within the Consolidated Balance Sheets. Realized shortfall tax benefits (amounts which are less than that previously recognized in earnings) are first offset against the cumulative balance of windfall tax benefits, if any, and then charged directly to income tax expense, potentially resulting in volatility in our consolidated effective income tax rate. We calculated a cumulative memo balance of windfall tax benefits from post-1995 fiscal years for the purpose of accounting for future shortfall tax benefits.

Options may be priced at 100 percent or more of the fair market value on the date of grant, and generally vest four years after the date of grant. Options generally expire within 10 years and one month after the date of grant.

Information on stock option activity follows:

Stock-based compensation expense related to stock option awards was $26.8 million in fiscal 2011, $34.4 million in fiscal 2010, and $40.0 million in fiscal 2009.

Net cash proceeds from the exercise of stock options less shares used for withholding taxes and the intrinsic value of options exercised were as follows:

Restricted Stock, Restricted Stock Units, and Cash-Settled Share-Based Units

Stock and units settled in stock subject to a restricted period and a purchase price, if any (as determined by the Compensation Committee of the Board of Directors), may be granted to key employees under the 2009 Plan. Certain restricted stock and restricted stock unit awards require the employee to deposit personally owned shares (on a one-for-one basis) during the restricted period. Restricted stock and restricted stock units generally vest and become unrestricted four years after the date of grant. Participants are entitled to dividends on such awarded shares and units, but only receive those amounts if the shares or units vest. The sale or transfer of these shares and units is restricted during the vesting period. Participants holding restricted stock, but not restricted stock units, are entitled to vote on matters submitted to holders of common stock for a vote.

Information on restricted stock unit and cash-settled share-based units activity follows:

The total grant-date fair value of restricted stock unit awards that vested during fiscal 2011 was $93.6 million, and $26.1 million vested during fiscal 2010.

As of May 29, 2011, unrecognized compensation expense related to non-vested stock options and restricted stock units was $170.7 million. This expense will be recognized over 19 months, on average.

Stock-based compensation expense related to restricted stock units and cash-settled share-based payment awards was $141.2 million for fiscal 2011, $131.0 million for fiscal 2010, and $101.4 million for fiscal 2009.