I do not see this happening, so long as we remain completely earthbound and divided politically and socio-politically. It is hard even traveling to another country sometimes, so I think the one world idea is very far off, unfortunately. Perhaps once we colonize other worlds, if we do, this will change.
Humanity will never transcend the sovereignty of nations to become one world. This is because of the fact that mankind is simply a very selfish and destructive entity in it's own inherent nature. We are advancing things too quickly to catch up with them and we will end up destroying ourselves before becoming harmonious.
I really do not think we'll ever reach a point in history where we'll have a global government. And this is a good thing. We are a very diverse world, with people who have different needs and customs. One government could not possibly represent all people in an adequate manner.
The current trend sees many groups wanting to break off from their bulky governments, so essentially we are moving away from the idea of larger governments, from what I see. I do not believe we see a time in the foreseeable future where one government will rule. I believe this idea is scary to most and given the number of nations of this planet, it is very unlikely.