Check in the bios under system security settings. The power button options are there, it should be enabled by default. Also this is where the power recovery options are. It could be possible that the power button is set to disabled and the ac recovery is set to on, so when you pull the power to shut it off and plug the cable back in it starts up again.
So I have 4 Poweredge SC1435's I use as a vmware lab. I upgraded them all to the most recent BMC and BIOS before upgrading the memory and to Dual Quad Core processors. I'm seeing an issue with one of the systems after upgrading that when I plug in the power cord, the machine just boots up.. If I hold the power button it will not turn off, the only way to turn the machine off is by pulling the power cord from it. This was not occuring before the BMC upgrade.  One thing I did notice was that during my BMC upgrade, the machine powered off in the middle and I had to re-complete and force upgrade the BMC (all is well and reported okay while booting BMC is upgraded to most recent version) however the power button still does not work. I replaced the power button module including cable that plugs into the motherboard with a known good spare I have here and it didn't work either. (As a note, the one I removed from the machine having issues powering off worked fine in another machine as well).  Any suggestions here?  Anyone seen this behavior? Thanks!Mike
Dell Poweredge SC1435 - Power button problems after BMC upgrade.