iPhone Inverse Display and Battery Life

[Timoleon]

I've done bit the bullet and just purchased me an iPhone (yesterday). I've downloaded Stanza and am overjoyed at how it plays well with Calibre. I'm transferring a bunch of ePubs over to my IPhone from Calibre, and I started reading a novel on the iPhone in bed last night on a lark (Sony PRS505 at hand in case of an emergency ).

I tried reading with the traditional black text on white background, which was fine. I then got to playing with the Yin-Yang icon in the Stanza controls, and tried out white text on black background. I little disconcerting at first, but I probably could get used to it. My question is: Does the white text on black background demonstrably extend a battery charge on the iPhone, or is it a myth, or is it too little to really matter?

As a crass newbie with an iPhone, I've gots to know!