There was a study recently that said people are less likely to achieve their goals when they tell other people about them.
It's interesting that Robert Cialdini says the opposite; according to his concept of "Commitment and Consistency", if people commit, orally or in writing, to an idea or goal, they are more likely to honor that commitment (http://en.wikipedia.org/wiki/Robert_Cialdini#Six_.E2.80.9CWe...).