Only connecting AREF to 3V doesn't work without doing something to the code. My AREF pin is 5V on its own, but when I connect the 3V it does nothing and actually 'overrides' my 3V bus so that anything else connected to it now sees 5V.
According to Massimo:
The aref works like this...
When you do analogue to digital conversion the arduino board uses the 5V supply as a reference. this means that any voltage between 0 and 5v is represented as numbers between 0 and 1024. If the signal you are measuring never reaches 5V it is possible to change the reference to an internal 2.5 v source and this divides the space between 0 and 2.5v in 1024 steps effectively doubling the resolution. If this still doesn't work for you then you can ask the processor to use an external voltage applied to aref. if you find a super stable 1V refernce source this will give you 1milli volt resolution for the ADC. this is used only in special applications because at that resolution the noise becomes a big issue etc etc
switching reference voltage requires adding a few lines of C code to your arduino code.
Summarising : changing your aref is advised only if you have good grasp of analogue electronics otherwise it's a mess..
Now if only I could figure out what these lines of code are.