P4M800Pro mobo won't power up...

I've installed my new P4M800PRO-M (Version 2.0. And it's part number
15-Q22-012022) in a new case (with CPU, memory and all that stuff) but
pressing the front panel "On/Off" switch won't bring up the PSU. But if I
manually jumper the "PS_On" lead to ground up it comes (at least the CPU and
case fans start and the all voltages come up).

I've also measured at pin 6 of the front panel header and can see the power
switch applying a ground there when I press it. But Pin 16 on the ATX-24
PSC harness connector never goes to ground to signal the power supply to
start.

Any ideas?