Something to maybe add to the Known Problems.
Calendar App *can* cause massive battery drain
hxxp://forums.hardwarezone.com.sg/showthread.php?t=2792427 (couldn't remember if links were allowed)
I don't take any credit at all for this...I had unbelievable drain...googled around, found this and my battery went from struggling to last 9 hours with moderate use to 24 hours with fairly heavy use.
It seems that the calendar app (not the widget) keeps running and refuses to let the phone sleep properly.
Goto the dialler.. type in:
From the menu that appears, go to Battery History, then change the first drop down to be "Partial Wake Usage"
If the calendar app has the largest (and full) bar...you have the issue.
Force stop the calendar storage app in Manage Applications and then relaunch it manually (I don't have to do that as I have the widget running...I just view the widget)
Since then, my battery has been superb