There is an article at AOPA (http://flighttraining.aopa.org/students/presolo/special/medical.html) that says you can fly if you don’t have a medical, providing that there is a qualified pilot on board who is appropriately rated, current, and knowingly acting as pilot in command.
I looked in the FARs but I cannot find any regs that specifically state this. Does anyone know if it is specifically stated anywhere or is it just implied?