The IRS is very picky about what constitutes an independent contractor. This has long been abused by companies trying to skirt employment law and avoid payroll taxes. They have a litmus test of sorts to determine whether someone is an employee or an independent contractor (tl;dr: if you treat them like an employee, they're an employee). And of course, there are significant penalties for getting this classification wrong.
My lawyer (a tax attorney) said this is rarely prosecuted on independent contractors who are themselves incorporated (along with things like actually have a contract etc). From my own experience this is true as well. Think about it, the IRS would have to give back all of the matching that the contractor (agent) did when they collect from the employer (principal).
For more details, go to the source: http://www.irs.gov/businesses/small/article/0,,id=99921,00.h...