I notice that the battery life of my phone can last longer when it is in GSM (2G) mode than WCDMA (3G) mode.
Here is a simple observation of its battery consumption pattern. I switched it to GSM mode (Settings > Wireless & networks > Mobile networks > Network Mode > GSM only) during day time, and switched it to WCDMA mode (Settings > Wireless & networks > Mobile networks > Network Mode > WCDMA only) before I slept, and switched it back to GSM mode again the next day. Along the whole period, there is no phone call.
Here is the battery consumption pattern I get:
and here is the same graph shown in landscape view:
It is very obvious that when the phone is on WCDMA mode, its battery consumed faster.
This is an HTC Incredible S upgraded to Gingerbread Android (version 2.3.3). I do notice that when it is in GSM mode, the signal is pretty stable (I can always get a full bar), and when it is in WCDMA mode, the signal is not that stable (there are frequent occasions of losing one bar or two bars). Probably the stability of the signal is one of the major factor in battery consumption.
Anyhow, it is a known fact that the phone will consume less battery when it is in GSM mode rather than WCDMA mode, and it is among the tweaking of battery saving apps such as Juice Defender.