IRS Summertime Tax Tip 2009-20 lists "the top ten things every business owner should know about hiring people..." and related links to IRS Publications and additional information. Critical factors affecting classification of workers as employees vs. contractors include the type of control the company has over the individual's work and the nature of the business relationship as understood by the parties involved. (Other factors not listed in this notice include who bears the risk of loss, etc., but this notice is a good starting place.)
As always, please give me a call to discuss the particulars of your specific situation.