There's an interesting post on DisplayDaily, trying to compute how much more efficient will an OLED TV be, when compared to the newest LCD TVs?
OLED TVs power consumption is supposed to be around 70% less than a 'plain' LCD. But the newest LCDs use a LED power source, and a new technology called Local-Area-Dimming (or LAD) actually allows the TV to use less energy in sections that use less color. This also gives the TV better contrast (black will finally be black).
According to the calculation of DisplayDaily (which is more of a guess, since we don't have large OLED TVs, and we do not have any shipping LCD with LAD yet) - an OLED TV will still take 30% less power than the best LCD. That's still very good!