Clean skin often has a resistance of about 500 ohms. Dirty skin can have electrical resistances of up to several million ohms.
Your body resistance varies greatly in different parts of your body. A value of 1500 ohms is commonly used as the resistance between major extremities of an average human body: hand to hand, or hand to foot.
Also, assuming PaulS's cicuit works, I am concerned about something in the design. When you place your finger across the terminals, they have a lot of resistance, and things are hunky-dory. But what happens if a piece of metal accidentally comes in contact with them? Then there's no resistor between the +5v and the pin of the Arduino. Is that a problem? I'm not sure, but I think that would let hundreds of milliamps of current flow and would fry the Arduino.So to prevent the Arduino from being fried, I think I should use one of the following setups. I suppose it doesn't matter which, since R2 would only be 1K or so. Though perhaps it should be larger than that?
Another thing that concerns me is how much current will be flowing across your finger. I saw something that said 10mA is painful, and 40mA is dangerous. But is that a concern with a battery? I don't know. But I could select the size of R2 to get the mA down into a safe range if that's an issue. I suspect it isn't though.
When no connection is being made, do you suppose the Arduino will return 0 for the value on the analog port, or might I want to treat say, any value less than 512 as an off state? I suppose I may need to do debouncing as well because this switch will need to toggle an action. There's a debounce library right? I'm sure I saw one.
Not sure why the setting is called 2K when it's obviously reading 1K as 1.0, but nothing in electronics ever seems to make sense to me.
Still not really sure why it's not 10K though. 9.9999 would still make more use of the display than 1.9999.