Employers are not required to reimburse the mileage driven by employees in the course of employment unless the employee is working under some sort of an employment contract. The mileage rates that the IRS post are maximum reimbursable amounts that are allowed to be excluded from taxable wages. 

Also as of July 1st, 2008 the IRS raised the maximum allowable amount up to 58.5 cents per mile for business purposes.