Interstellar colonies will only be attached to Earth as long as they are dependent on it. Once they are self-sufficient, their seperation is inevitable. The status quo may be maintained for quite some time if some smart leaders are in charge, but eventually some minor issue (tax on tea) will be enough to inflame them.
This does not mean, however, that some interstellar governing body is not possible. Anything that does form will be like the Commonwealth of Nations, with Earth as Britain. But, before then, it's all too likely that there will be outright wars of independence as Earth-based political bodies attempt to maintain control over their colonies, with attempts to give more self-governance to colonies coming only once there have been some devastating examples of colonies breaking away.
The problem with a Commonwealth, then, is that Earth will be the center. Earth-based political entities will not be willing to accept anything less. Earth's eventual decline in importance, and the development of the now self-ruled colonies, will see the power balance changed, but no reform will take place until there's been a large and public demonstration of the change in the balance of power.
The nature of this demonstration, whether it's from internal or foreign trouble, will affect the result. If it's foreign, you may see the Commonwealth banding together, and the military commitment of the colonies demonstrating Earth's declining influence over them. If it's internal, then Earth will either accept the new order, or, more likely, take steps to increase control, which will either result in a weakening of the colonies and the Commonwealth as a whole, or a reaction and more power to the colonies, possibly with the independence of some.