An article in today's New York Times addresses battery usage in toys and other electronic devices. It also features photos of the NXT. It states that:
1) The primary drain on batteries is external heat (naturally).
2) Rechargeable batteries suffer significant power leakage, whether being used or not.
3) Alkaline batteries have less power leakage and last longer.
The article goes into detail about all types of battery usage and is well worth the read. Find it here.