Hey CrossRoads, you're awesome!
I've done a bit more experimenting... When in full sleep, the chip itself seems to take almost nothing (maybe 0.1mA or something). The really cool thing is that it seems to leave it's output stages enabled, so that means any pins left as outputs, and set to 0 or 1 stay that way when it's asleep (this is cool because I can set one axis of my touch screen up for reading, and have an op-amp provide me with the hardware interrupt).
I'm now looking at dropping the chip down to 8MHz - I tried to do it by bootloader/fuses etc, but for the life of me, couldn't get it working properly (it just refuses to be programmed) - that's another problem. I've done something similar by using the clock pre-scaler:
CLKPR = (1<<CLKPCE);
CLKPR = 1; // this is actually "divide by 2"
(Which I found here: http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1163418637 - this messes with delay() and millis(), Serial and pretty much everything else)
Anyway, the good news is that at 8Mhz, the chip takes about 75% current it does at 16Mhz (I measured 27mA at 8Mhz, versus 40mA at 16Mhz).