The IRS has issued a Fact Sheet (FS-2007-27) on Employment Taxes and Classifying Workers:

Being an employer carries important responsibilities. Critical issues include making sure all workers are properly classified as employees or independent contractors and making sure taxes are withheld and paid in a timely way. This fact sheet, the 19th in the Tax Gap series, helps business owners better understand their responsibilities regarding employment taxes and classifying workers. It is a common misconception that someone working part time or earnings less than $600 per year should be classified as an independent contractor. But in fact, part time status and the number of hours worked are generally not factors that determine whether a worker is an employee or independent contractor.