The vote in Iraq on whether or not to accept the Constitution might very well have a result that the Constitution in its present form is rejected. That will not mean the vote failed. The vote only fails if it doesn't happen, or the result is somehow made meaningless.
If the Constitution is accepted, then the Iraqi people will have spoken and said, "Yes, that is acceptable." The government will have gotten permission from the people.
If the vote is to reject, then the Iraqi people will have spoken and told their government, "No, that is not good enough. We demand better." That's the very core of representative democracy, that the people can demand that their representatives actually be representative.
Which will it be? We shall see. But no matter which way the vote goes, the important thing is that Iraqis will be voting and having a real influence on how their government will operate.