Someone did a battery-life comparison of several android handsets, and according to that test, handsets with AMOLED has the poorest battery life. LCD handsets are better (the Galaxy S, with its large Super-AMOLED display is in the middle). They tested mostly web-browsing, which means a lot of white, which is not good for OLED displays. Also of course the AMOLED display is not the only difference between those handsets.
AMOLED should theoretically offer lower power consumption, but this is not the first time we hear that newer LCDs are actually better. For example, when HTC replaced AMOLEDs with LCDs in the Desire phones (and others), they said that the battery life is better with LCDs. The good news is that Samsung plans to soon double the efficiency of their AMOLED displays. AMOLEDs are at an early stage soon, so we can expect fast advances in all aspects, including efficiency.