When you pressed the PMU button, did you follow all the correct procedures for doing so? I say this because there is a specific set of steps to follow when resetting the PMU... they're not at all hard, but each step is detrimental to whether the PMU gets reset correctly or not (ie, pulling the power plug completely out of the computer before pressing the button, pressing the button for the correct duration, etc.).
http://docs.info.apple.com/article.html?artnum=86760
As eric2006 said, a dead logic board battery can also cause this -- resetting the PMU uses power stored in the battery, and if the battery no longer holds a charge, then resetting the PMU cannot happen.