I started getting the login / sign in error when I changed sim cards whilst travelling to Europe and Hong Kong. So essentially the phone works fine but does not sync to my gmail or use internet very well. I would get a google-acct signin error often in my alerts.
After readng all the threads, i tried this one thing and viola - it worked.
When you get your samsung and set it up the first time, you would have entered your google/gmail email login and used a password to setup for the first time - what you may have done is use the Swype keypad input method (well, I had unknowingly done so) and hence wen u change ur input mthod default to samsung or qwerty later on - the login password does not work.
I changed my input method and typed the same password I had been trying for days and it actually worked. Then I switched back to samsung keypad as am not comfortable with the swype keypad.