Any ad units generated within the last 7 days that haven't yet received any impressions will now be listed as 'New', rather than 'Active', in your status column. This should help avoid confusion about your newest ad units and those already appearing on your pages.
You can now view the most recent date when each of your ad units was updated. Please note that these dates aren't retroactive, so any ad units you generated or edited before this display change will show a last-edited date of November 1st, 2007.