I found this tutorial here. http://www.prash-babu.com/
This small tutorial would help you fix the issue where checkgmail( which is an application which shows you all your latest gmail mails on your desktop) fails to login . The app repeatedly asks you for your username and password saying that you've entered the wrong username/password . So here's how you fix that :
Step 1 : Go to the Terminal(Applications->Accessories->Terminal)
Step 2 : Now type checkgmail -update
Step 3 : Ubuntu would now look for the latest update of checkgmail which would definitely have a fix for the bug
Step 4 : Once the update process is finished , restart checkgmail
Step 5 : Checkgmail should now login and work as usual!