PedroA:
Would the second circuit (please see attached) still work fine if a load such as a camera PCB (like the HackHD camera PCB, http://www.hackhd.com/index.php) was the load, knowing that it runs at 3.7V and about 400mA? Can the Arduino successfully turn it on or off via the mosfet without any issues?Thanks
Absolutely, that MOSFET has an on resistance of 65mOhm, 0.4A will drop only 25mV
or so. Start to worry about thermal issues from 1.5A upwards.