Ok, so I sort of drove near a black hole on my way to Radio Shack, and suffered from some time dilation effects...
I cut the cord off of a 360 power supply, and wired it up using the Picture above, and... Success!
The 360 does indeed turn on and off the PC PSU when you turn on and off the 360 Console.
Also, you can still use the Wireless 360 Gamepad to turn the 360 on and off.
It's pretty sweet.
Many thanks to the originator of this thread, as well as to all of the contributors.
Now that I have the wiring diagram figured out, and have confirmed the magical deliciousness of the recommended Transistor and Resistor from Radio Shack, all that remains is to find out what the model numbers are for the SMD equivalents for the Transistor and Resistor.
I'm gonna have some PCBs made for that custom case I'm designing (an internal PC PSU to XBox 360 Power Adapter), and want to make sure that I find the right parts.
I think that I found the part number for a SMD version of the NPN Transistor:
Is that the right one?
As for the resistor, I'm having trouble finding a SMD version of it.
Again, any help would be greatly appreciated.