Power corrupts, but an AI need not be human enough to have corruptible attributes. The increasing precision of formal legal language has met the improving fuzzy logic capabilities of AIs. Many groups are now experimenting with autonomous legal systems.
Writing a smart constitution requires a somewhat different approach than a traditional document; they have trouble with generalities and abstract appeals to justice. Even the smartest constitution tends to err towards literal interpretations, and will favor what was actually written over what may have been intended. They are an effective supplement to a government, but it is a risk to have them themselves govern.
A smart constitution is at its best when working within a formal, written framework. They therefore have served best in extropian influenced habitats, where formal contracts are the foundations of legal systems. Otherwise, they are most often used as hedges against power consolidation in autonomist habitats, carefully defining what can and cannot be done with formal power.
Inspired by Anders Sandberg's smart contracts.