A recent study of Milton Sousa shows that modesty plays an important role for powerful leaders. It shows that the more powerful a leader is, the more relevant it is for him or her to be modest. Successful and humble leaders benefit from a more open culture. On your way to the top, you do have to be aware though that modesty could not be the best approach.
Another article can be found here: http://intranet.rsm.nl/portal/page/portal/Intranet/old-intranet/news_events/Additional_Media_Material_4/Bescheiden%20leiders%20zijn%20effectiefst.PDF