Bunsen, I read somewhere that you are only ever supposed to press and release PMU reset buttons and that holding them down for a period of time is not recommended and/or could corrupt the power manager again. I believe that it was form Apple but I can't cite my source ATM.

PMU reset is a power manager reset, not a PRAM reset. Also, some Macs such as the IIci and even the 6100, don't have a PMU reset button. From what I gather, the button specifically fixes soft power issues.