We often write about the power saving nature of OLED displays, like Samsung's Super AMOLED displays, when displaying a primarily black interface. If you like bright colored backgrounds or surf the internet a lot on the web browser, which is primarily white, OLED displays actually consume more power. LG release a comparison of the power consumption of a it IPS display as against a OLED display.
On a side note, I was a bit surprised that an all black screen consumes as much power as when it is displaying a 100% white image on IPS display. I was under the assumption that a primarily black display would save some power regardless of the type. From the chart above, it looks like on a IPS display (and probably other types of LCD's too) power savings using a black background is negligible, if any.
Going back to the OLEDs display, having you phones user interface set up to have a primarily black background or darker backgrounds will extend your battery life.
One area where Super AMOLED phones fair poorly is searching the web, since most websites have a primarily white or light colored backgrounds. Looking at GSM Arena's battery life test shows how poorly the Samsung Galaxy Nexus fares with the web browsing.
In order to save battery life on a phone with AMOLED displays, I would recommend a black or primarily black wallpaper. Many apps also have a black background option. Another option for saving battery life while browsing the web is using Black Google Mobile app or doing a Google search through the Black Google Mobile website (just bookmark this link in your mobile browser). Black Google returns web results in over a black background.
Still when you finally go to the website you are looking for it will display the website in its default colors. So basically it saves battery life only while searching, and not while reading websites. Still, even a little battery savings is worthwhile.
Basically, if you have a OLED display on your phone, learning to love black will help you extend your battery life.