So I have an external power supply for my breadboard. I assume I run the 5V+ from the breadboard to the Vin on the arduino and ground from the breadboard to one of the Gnd pins on the arduino?
If this is how it's done is it safe to plug in the usb cable while the arduino is powered like this?
You don't need to power the arduino from the breadboard if you will have the usb plugged in.
So it won't hurt anything to have the arduino and the breadboard using different power sources/grounds?
Nope you can turn on the breadboard power and the USB power at the same time. The circuit will just have more current.
But, chances are, you will need to attach the two grounds.
That depends alot on your setup, if you use the bread board supply for anything that's attaching to the Arduino, you need to attach the grounds. 
Indeed, it's best to attack the grounds, however, you most likely don't need to attack the Vin from the breadboard, unless, as mentioned above, you want the extra current.
I will be using the breadboard voltage to power sensors/LEDs on the breadboard but I will be using my arduino to read the values from the sensors. So in this case I'd run a ground from the breadboard to the arduino gnd?
Yea, that's all you need.
Thanks guys!
Indeed, it's best to attack the grounds, however, you most likely don't need to attack the Vin from the breadboard, unless, as mentioned above, you want the extra current.
Attaching a regulated +5vdc to the Vin pin will accomplish nothing. It's too low a voltage to drive the on board +5vdc regulator and it's below the 7.5vdc needed to switch the automatic voltage selector on the Arduino board.
It's not a good practice to attach an external +5vdc to the Arduino +5vdc pin while also attaching the USB connection. You are effectivly wiring together your PC's +5vdc supply voltage with your breadboard's +5vdc regulated supply, and while it might work it could lead to problems both for your PC and/or your external +5vdc supply.
Best to just tie the grounds together and use your breadboard +5vdc to power external loads, but allow the Arduino board to be powered with just the USB cable. Or use a external supply of greater then 7.5vdc and use that to power the Arduino board via the Vin pin.
Lefty
Good point. That was my bad. Vin is unregulated power, 5v is the regulated power pin. Thanks for the clear-up retrolefty