I live in Germany and got my Nook the 3rd day now and I was curious.
So last night 22:00 o'clock I put him to sleep
Airplane mode

n
wifi : off
Battery: 80%
This morning 7 o'clock it lost exactly 3%. So that makes 3% in 9 hours. Guess that is ok.
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Now, this is realy extremly strange. I went to work this morning. So I wanted to know, how much battery the Nook will use, while he has Wifi turned on. Now, see the suprising result:
Airplane mode: on
Wifi: on
Battery: 74%
Sleep modus
I started this test at 8:45 and ended it at 13:45 so he was exactly for 5 hours asleep. When I woke him up and looked at the battery status it said 73%. Which would mean in 5 hours with wifi on and airplane mode on it used only 1%!!! Which would mean, it uses less than with wifi turned off. That is strange.
I start to think that maybe the 3G network is causing this problem. Over here in Germany I cannot connetc to the AT&T network and I never saw the network sign in the upper corner on my Nook. I know with my iPHone it was the same problem in the beginning. It used a lot of battery power trying to connect all the time to a network. So as long as you stay in the same wifi network the Nook is not busy trying to connect you to a different network. That is at least the only explanation I have for this.