Well, you're always going to get SOME kind of reading. It's actually not a very good circuit because with the transistor off, it's as if the transistor and R2 do not exist. So you have a path from 12V, through the 3.3k resistor, and into your Arduino. The 3.3k resistor limits your current but it's still not a good idea to apply 12V to the Arduino input pin.
I would not have predicted the 0.80V myself (I would have thought it was closer to the maximum possible reading). Is it possible your code is not designed to handle a reading of 1023 (maximum 10-bit ADC value) and prints out 0.80 erroneously?
The Gadget Shield
: accelerometer, RGB LED, IR transmit/receive, speaker, microphone, light sensor, potentiometer, pushbuttons