I've also noticed that I can get a value of 465, and upon increasing the light slightly (the light shining on the solar cell, which is connected to the ADC) the value changes to -460. Seems backwards to me.
What's more confusing, I use the same code elsewhere and it works fine:
long int runAverage = 0;
long int cellVoltage = 0;
runAverage = runningAVG(cellPin,5);
cellVoltage = (runAverage*1100) / 1024; // Convert reading to Voltage