I think you have to look at the definitions in the statute and regulations. If you look at the I-9 form it says:
The government has a handbook for employers that you can find on the web site at http://www.uscis.gov/files/nativedocuments/m-274.pdf . It says that you don't have to complete the form for an independent contractor, but you may not use contractors to circumvent the law related to the hiring of undocumented workers. I'd be careful though. If a contractor doesn't meet the definitions for independent contractor under the wage & hour, and tax laws, they might be considered an employee and you would have to complete the I-9.