OK, I've got a P channel FET (not my design, just took it over) that is using 3.3V to switch another 3.3V supply (always on turning on the rest of the board).  Part is a FDC638P.  Looking at the data sheet, Vgs is the voltage differential to turn on the gate, right?  So pretty much....you can't have the same voltage on both the gate and drain to turn it on, just wont work.  Now if I had 5V (or anything more than 4.1V) there, with a resistor to ground to keep it turned off, that should work.
 		
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