I too have some problems considering the screen on time on ICS 4.0.4 stock. Got the OTA about easter and I noticed a heavier battery drain when im not actually using the phone. No apps run in the background, nothing is syncing, 2g networks only and my phone still loses ~1 - 2% of charge an hour.
But then again, it does occasionally lose only 1% through the night. (See attachments, sometimes its flat, then again steep when im not acutually using it)
Back when I had GB, I got at least 3.5 - 4 hours of screen on time with the same usage pattern. I might still get that screen on time, but only if I use it continuously. Also on GB, the battery usage of Android-System was never as high as on ICS and again, I have the same usage pattern.
I'm out of ideas, but the battery drain ruins the whole phone!
Does anyone knows something to solve this?