I had a 34 hr labour with my DS... we didnt know at the time, but 30hrs into it i was only 6cm dialted and the midwife broke my waters, his head was in an odd position, so when the waters were broken his head turned the way it should have been... contractions got way way way more intense, and labour was all over 4hrs later

so it could have jsut been the pressure from his head being in the correct position that hurried things up - or it could have the the waters being broken, or a mixture of both...

But i do believe that breaking the waters can speed up labour, but not as much as its made out to be...