Are you guys discussing this battery problem on iPhone 4S or iPhone 4?

almost all day when I'm in WiFi range. Otherwise I will switch off WiFi.

It's known fact that WiFi uses less battery juice when compared with 3G simply because WiFi range is smaller and you are usually in a very close range to the access point (AP). In contrast, 3G coverage is large and its dependent on the closest repeating telco tower that you are currently in. In other words, if you have less coverage (low signal bar), GSM module in your iPhone will be transmitting at the max power. Max power is needed in order to secure connection with the tower. As such, your iPhone battery will drain faster and at the same time hotter.

So my advise would be to always use WiFi whenever possible. Try not to use your iPhone or any other phones in the area of weak signals as the transmission power is the max at that point of time.