I had the same power button issue with my Captivate as well... and thought that issue had been looked into. After 14 months of use on my S3, I noticed this issue appeared.
Solution (well not quite): change key layout to disable power button. Re-map volume down as "power, wake". Yes, by doing this, I am losing volume down button. Also, when the phone is off, I still need to press the power button to power it up. My fix of changing the key layout file won't take charge until after the phone boots up.