How frequently do you login into the Google Adsense website to check your income from visitor clicks ? Once in a day or once every hour or probably you have set your Firefox Adsense plugin to retrieve the statistics every five minutes.
Google is now displaying this information for you at the top right corner in your Adsense control panel. It tells you when did you last login into the system. The online bank accounts display this information too since t is a good security measure.
What if you return from a week long vacation and Adsense tells you that you last logged into the system 6 hours ago. It's time for you to investigate and change the password.
On a related note, a lot of us suffer from the "check-income-every-minute" Adsense syndrome but I this such activities just distract you from current tasks. You should probably focus more on writing good content and the earnings would increase automatically even if you don't check them every two minutes.