100uA is still pretty high. It really gets interesting once you go below 30uA. Depending on what you do you may even get below 1uA.
The canonical example for low power Arduino seems to be the nightingale example http://interface.khm.de/index.php/lab/experiments/sleep_watchdog_battery/.
You may also want to look here:
http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1273507808
http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1278021334
And here
Power Switch - Mikrocontroller.net (in German but the code comments are English) http://www.mikrocontroller.net/attachment/81946/PowerSwitch20100711.zip
The setup of the last example will bring the controller down to 0.4uA while sleeping and 214uA while running.
With regard to unused pins:
- You must ensure that no pin floats. Floating pins will increase current consumption.
- It does not matter if you pull them low or high. Unless of course there is some external component connected to it. Then it does matter. Depending on the external components (e.g. external pull up or pull down) it may depend.
One thing though: being able to run with 0.4uA does not imply being able to start with 0.4uA. During startup the AVR will always draw significantly more current (>>10uA). This is the "over the hump" problem that prevents starting from very high impedance power sources unless you add some additional components for power control.