Some Galaxy Nexus users reporting odd battery readings
There have been various reports that the battery monitor on the Nexus won't see much of a dip, but then will drop fairly quickly once it's past the first 20% of battery use. Additionally, the last 20% can often last a very long time. There have also been reports in the MoDaCo forums about large dips on the battery monitor overnight, as if the phone isn't going into sleep mode properly.
Of course, these are all unofficial reports, and don't even come from official Google support boards, but it does seem there is something wonky with the battery meter on the Nexus, or possibly the radio not sleeping properly. It will likely be fixed in an update, and it shouldn't stop you from getting a full day's use out of the Nexus even if the reading drops quickly.