From IRS 'Instructions for W-2 and W-3':
"Code V—Income from the exercise of nonstatutory separate statement to your employee. You may also use this stock option(s). Show the spread (i.e., fair market value of stock over the exercise price of option(s) granted to your employee with respect to that stock) from your employee’s (or former employee’s) exercise of nonstatutory stock option(s). Include this amount in boxes 1, 3 (up to the social security wage base), and 5.
This reporting requirement does not apply to the exercise of a statutory stock acquired pursuant to the exercise of a statutory stock option.
For more information concerning stock options, see Pub. 15-B."
Source:
http://www.irs.gov/pub/irs-pdf/iw2w3.pdf