Hmm, this is a bit of an odd one.
I've recently built a new i7 920 system with an Asus P6TD Deluxe motherboard.
It's just been built and has been running for a few hours okay but I noticed something strange. When the PC was powered off (though the PSU was receiving power & still had the power lead into it) I decided to rejig the placement of a couple of the SATA leads.
When I unplugged the SATA connector (this is the data lead, not the power lead) and put it into another socket, the PC powered on. I've tried this a couple of times in case I'd done something else but no, putting a SATA lead in (to at least two of the sockets at any rate) seems to switch the pc on.
I'm not keen to repeat the process in case it's a short or something similar but having said that all stand-offs look good and I can't see an obvious fault anywhere. The system seems to be running fine otherwise too...
I've not experienced this before - perhaps I'm being a total noob here - anyone have an idea as to what's going on?