For now I've switched off the functionality again to be able to hang up using the power button, but still it's quite frustrating.
On the other hand, a combination of updates and removal and deactivation of some apps, maybe Google's Listen app, provided a huge amount of power savings. Now my phone - a Samsung Nexus S - suddenly lasts for over two days if I don't use it a lot! It's never lasted this long. And I love it, naturally. Maybe Better Battery Stats and other apps, probably the Better Wifi On/Off helped, too, with that.
Especially I'd really recommend Better Wifi On/Off, as it's free and pretty much exactly what I'd always been looking for. Without lots of resources, if switches Wifi off if the display is off and no network traffic is active. But it turns wifi on when the display is unlocked or the device is connected to power. That's all very sensible I think. Now if it could also switch off 3G when wifi is connected... I'll ask the developer. Maybe he can make it work.