gr0p3r:
What sort of battery life would you expect if i did away with the regulator and its 2 capacitors, hooked up it directly to 4x 2500mAH rechargable NiMH which are 1.2V fully charged? Assuming i can get the sleep code working too. know how to wake it after its sleeps, and is the sleep method from the nightingale sketch the way to go?
If you use regular NiMH cells, they will lose their charge through self-discharge in a small number of months. If you use low self-discharge (also called hybrid) cells, they should last longer than a 9v battery, probably several years if you get the current consumption low enough. The starting voltage of a fully charged NiMH cell can be as high as 1.4v so you may briefly be running the processor at 5.6v.
Another possibility is 3 x lithium AA cells. These have a long shelf life and flat discharge curve, see http://data.energizer.com/PDFs/l91.pdf.