[glow]>you don't want to put 9v directly on the pin[/glow]
This means don't connect 9 volts to any arduino digital or analog input pin
[glow]>the maximum is 5 volts for a board that is running on 5 volts![/glow]
The maximum voltage you should connect to these pins is the voltage powering the controller chip (usually 5 volts)
[glow]>If you want to measure 9v you would use a voltage divider. /glow]
If you use a voltage divider to divide the voltage in half (using two equal value resistors) then the maximum value from analogRead will actually be 10 volts ( you get 5 volts on the pin because you have given 10 volts into the voltage divider and that has divided the voltage in half)
[glow]A search on voltage divider should yield more info on this [/glow]
I hope that (and a read on some of the hits from google ) will help.
Here is one of the threads where this has been discussed: http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1226005458
Have fun !