That's a good question. The law is pretty complex and I don't pretend to understand it all, but the basic answer as I understand it is that piece work rates have to be calculated in such a way that you'll get the "prevailing wage rate" -- http://www.dol.gov/elaws/esa/flsa/14c/18e.htm

There are also exceptions for people with disabilities, special minimum wages (you can actually pay the disabled less than minimum... sometimes), etc.

So, I think the basic answer is that you still have to respect minimum wage, but how you do that is pretty complex.