this is not mentioned anywhere in the documentation as far as I can see.
The question is: do the 2 extra analog pins (A6 and A7), which are found on the Arduino Nano and similar boards, have internal pull-ups?
I had a look at the ATmega datasheet, and it makes no mention of pull-ups for the two corresponding pins (ADC6 and ADC7, cfr. paragraph 1.1.9). It explicitly says, however, regarding the other pins (cfr. paragraphs 1.1.3, 1.1.4, 1.1.6) that they are "bi-directional I/O port with internal pull-up resistors".
So my guess is that there are no internal pull-up resistors for ADC6 and ADC7. Google also reports this explicitly mentioned on some AVR forum (http://www.avrfreaks.net/index.php?name=PNphpBB2&file=printview&t=89804&start=0).
Is this true? Otherwise I must conclude that my board is faulty :-)
If this is the case, BTW, it would be nice to add this information on the Arduino Nano page, in order to spare some possible headache to other hobbists...
Some background and context: I'm using the analog pins to read an array of buttons, similar to what described here: http://arduino.cc/forum/index.php/topic,20125.0.html.
To keep the circuitry at a minimum, I turn on the internal pull-up by using:
pinMode(A6, INPUT); digitalWrite(A6, HIGH);
The trick worked so far with A0 and A5, but it doesn't with A6. I'm getting wildly fluctuating readings with no buttons pressed, which suggests the internal pull-up is either broken or... simply non-existant :-)