adwsystems:
The photocell and resistor are in series. But the resistance of A0 to ground is in parallel with the resistor. The resistance from A0 to ground is 100Megaohms (that isn't a made up number, it actuall is 100Mohms per the datasheet), much larger than your 10k resistor, so in essence no current flows into or through A0.Just remember, path of least resistance.
At room temperature the input pin resistance is probably more like 10^10 ohms. The datasheet value
is the worst-case for all temperatures and supply voltages. You can assume inputs take no current.