One of the thorniest parts of a United Earth will be getting all of the countries to join such a world government ...
There would have to be a compelling reason to unite under a single government, other than simply just to do it. Today nations can enter into multiple international organizations, trading blocks, defense agreements, etc. they don't have to become one government to receive those benefits.
Fifty years ago there were 163 sovereign nation on Earth, today there are (debatably) 195. The current move is toward more nations, not towards a single worldwide state.
I read something on the Venus Project website that said in order for this to work effectively, all countries will have to pitch in, otherwise we might fall short on the amount of resources available to distribute.
To me it kind of sounds like they (TVP) don't want there to be a "competing" system running side by side with theirs. Something that could be used to comparison test their system, and it's performance.
It would be something like with the peoples in eastern Europe during the cold war. They were situated between two competing economic systems (and forced to live under one), when they were finally able to choose which system to live under, the choice was an easy one. They picked the one shown to actually work in the real world, and not the one that worked in theory.