Well, in theory if the rent charged is less than "market price" (whatever that means) then it can be considered income.

In practice, it is hard to prove that some rent is below market rates and from a bureaucratic point of view, the amount of man hours the IRS would have to spend to prove that you are getting unfair income is pretty substantial. 

The bottom line is that as long as you pay a rent that is even vaguely reasonable you are ok. You will only be vulnerable in an audit if the rent you are paying is egregiously low (like less than 10% of market value).