All employers that have employees or contract workers are required by law to give their employees and/or laborers their Form W-2 or 1099 by January 31, 2011. This can be done either in person or by mail.
If you are unable to reach an employee, you must mail their form to the address they provided to you. If it the form is returned to you the IRS requires you to keep the copy of the form for 4 years. You should keep the form in the envelope as proof that you mailed the form by the deadline. If you cannot prove you attempted to give them the form, you can be subjected to penalties and interest.