Crashwg has said it all
An ATX power switch is a standard normally open momentary switch. You close the circut once and it goes on, you close it again and it goes off.
So any standard arcade button will fill this role just fine.
On a totaly unrelated note, the fact that the ATX power button is a "stateless" momentary switch means that there is power running through your PC regardless of if it is on or off. for this reason it is not always a great idea to swap AGP video cards unless your machine is off at the wall.
Also, on some older keyboards this results in the caps/number/scroll lock LED's still working when the machine is off. Not 100% sure if that little gem will be useful to anyone, but it's too good to keep to myself.