How many pics are you getting from using rechargeables and what type of rechargeables are you using? I think I'm over 2000 pics on one charge and still see full charge in the top LCD. I shot alot of my kids doing sports, and their friends for one week without taking out the batteries and recharging. I'm using the Kodak 2500 MAH.
Energizer 2500mAh, well over 1000 so far on the first charge, so I don't know how far they'll go. Others have said around 1400 is to be expected. BenV makes a good point on flash: I use mostly SB-800 flash and very rarely touch the built-in except for CLS.
The only thing you might not know is that the rechargeables don't communicate their charge status as well as the Nikon batteries, so you're likely to go from seeing a "full" charge straight to seeing an "almost empty" charge at some point.