I'd like to use it as an analog input for a switch. From what I've read and been told it must not drop below a certain voltage. Firstly, is that statement true?
If so, what is that voltage? I'm confused as to what the data sheet is actually defining.
Look in the section "Electrical characteristics->DC Characteristics" on the datasheet. It's about 0.3V, depending on your Vcc.
Secondly, how on earth would I connect one up and what would it look like in the sketch? Would it just be like any other switch? I assume if it were to input at 0 @ 0v (open) it wouldn't even reset? And if it were to switch and send input value 1 (or 255) @ 5V it would be fine?
The reset pin internal pullup is about 45kOhms (it's in the same section of the datasheet). If you add an external pull-down resistor of double that, the voltage on the pin will be at about 3.33V (assuming you're using 5V). Use your switch to connect the pin to the 5V line when you press it. Measure the voltages using analogRead().