I've got one of those Nokia displays I've played with a bit but I haven't measured current draw; what do they pull?

I started messing with OLED displays and I was surprised that the one I have (128x64 res I think) only pulls about 20ma. That's way too much for an LCD unless the backlight is on but for an active display it's pretty stingy. Hopefully you can turn the backlight off on yours.

You could probably drop the clock speed quite a bit on that unit and still run just fine. Dropping down to 4MHz saves a lot of power on ATMega chips.

Good call on using an interrupt for the sensor, at high speeds polling would totally miss it.