Haha, yeah, I've got a bit of a headache in a very specific spot too
When I started playing Pilotwings, I had full 3D, and it was AWESOME, but after a while, I had to tone it down. I really love that slider.
I was able to get in about 3 1/2 hours after a full charge before it started flashing red. During that time, I had brightness at 3, power saving mode off, Wi-Fi on, I played all the built in software, a few DS games, and kept switching between 3D and 2D. Yeah, about what I expected. if you turned off 3D, wifi, turned on power-saving mode, etc, you should get about 5 hours.
3D pictures are awesome. Looking forward to taking 3D movies!
EDIT: You can turn off 3D permanently through parental controls. The system made a couple clicking sounds, the 3D light turned off, and moving the slider does nothing. I have a feeling that doing this could also extend battery life. I mean, now it's not doing double rendering just in case you turn 3D on.