This excerpt taken from the RVBD 8-K filed Apr 30, 2009.
Restricted Common Stock
Employees are able to exercise options before those options are vested. The shares received upon early exercise are restricted subject to vesting conditions which match the original option vesting schedule. The Company has the right to purchase back from terminated employees, at the original purchase price, some or all of the unvested common stock at the date of termination. The cash paid for the exercise price for these types of exercises is considered a deposit or a prepayment of the exercise price which is recognized by the Company as a liability until the restriction lapses. Furthermore, these shares are not considered issued for accounting purposes until they vest. A summary of the status and activity for restricted stock units under the Plan for the year ended December 31, 2007 is presented in the following table: