Since the theme is one government world and related ideas, I'd like to post my two cents worth. I think that a one world government would work, based on a few provisions.
First every ethnic group, religious sect, etc. would have to have their own planet. That way they could set up a system of laws and government (be it socialist, capitalist, democracy, communism, etc.), a constitution, and what type of police force they would want or have need for based on their ethos and whatnot. Now this may sound racist or separatist or whatever, but it could possibly cut down drastically on the wars fought between for example, the different sects of the Moslem faith, simply because of the cost involved of lauching an interstellar war and the factor of landing ground troops. If people wanted to live on a certain planet of a certain faith or ethnticity(sp?) they could. Its the ultimate in freedom of choice.
Second, there would be no government that controlled more than one world. That way for example a communist style world wouldn't be beholden to a capitalist body of government that ultimately controlled things beyond the planetary borders. There could be an interplanetary form of currency (to keep trade stable and profitable for everyone), a court system made up of equal numbers of members from each world (to keep things fair), and a whole lotta hope.
Third and finally, I know this sounds really farfetched, but maybe someday after we mature enough as a species, it could happen. Of course, alot would rely on FTL transport and the number of worlds out there (neither here nor there, at least not in this forum).
Before something like this happens, a single unified government is not possible on this planet. Humanity is simply too immature and warlike for something like trusting each other and putting aside old predujices and feuds.
Alot of these ideas (some were my own tho) were taken from a series of books by Peter F. Hamilton called "The Reality Dysfunction". I thought they would be somewhat relevant and wanted to share it and my thoughts.