The time has not come yet for a 'global' government but it will happen at some point. (Unless, of course, we destroy ourselves through nuclear and/or environmental self-destruction).
However, it will NOT be the result of GOVERNMENTS signing away sovereignty, it will be in response to gloabalization by PRIVATE multinational CORPORATIONS. They already operate on a global level caring nothing about nationalism or nationalist aspirations. The advancement towards a one-world government IS occuring and there is nothing we can do to stop it. But hey, nationalism has had a good run and all things must die.