I feel like the slow kid in class. What are all those diodes for?
edit: Oh.. so you are trying to power the arduino by pressing ANY button?
There's one major problem to watch out for. Depending on what you have connected to the other pins, it may not be possible for the AVR to actually disconnect from the ground rail, the way you have done pic1. The IC can ground itself through any other I/O pin that is grounded, because of the ESD clamping diodes on each I/O pin.
If that last sentence doesn't make sense: draw a diode between each I/O pin and the micro Vss pin. Anode on the Vss pin, cathode on the I/O pin. This should look really, really familiar.* There's another diode between each I/O and the Vdd pin. Anode on the I/O pin, cathode on the Vdd pin. (preceeding part of this paragraph edited for cathode/anode dyslexia). When your Vdd and Vss pins are connected to a PSU, this means the voltage you put on a pin should be within rails plus an extra 0.3 is ok. When the ground rail of the micro is not connected, this means any pin that has a path to the battery ground will feed the ground rail minus a bit of voltage drop.
*All those diodes you put in there are potentially redundant. They are already in the chip (very low voltage, low current, low Vfd shottky diodes, anyhow). So long as your current draw was very small, anyhow, you could do it without external ones. Provided there's no other path to battery ground in the rest of your circuitry. Basically w/e you made up with those rectifier diodes is already on EVERY I/O pin, not just where you put those external diodes. So plan (or abandon this plan), accordingly.
In most modern micros, you can probably use sleep to reduce current draw to almost nothing, but can you can do it in Arduino sketch? And also if you must use the onboard regulator, that is going to have a quiescent draw all the time. So sleep may not be practical for OP.