Also remember folks: by default the avr's (at least in the arduinos) use the 5V line as a reference for the ADC (0V = 0, 5V = 1023), but if your 5V is not too good or stable (e.g., using batteries directly hooked to the AVR not through a regulator) you can also use the INTERNAL BANDGAP REFERENCE (I believe it is 1.1 V).