Moppie, I too have been looking for the real answer for the EXs with optical slave triggers. I read many "guess" anywhere from EX's circutry are not designed for optical salves, to optical slaves do not drop the voltage low/high enough, or EXs have diffrent polarity. None of these info could be comfirmed.
I do like Lon's explaination of how we have "hard contacts" by physically shorting the center pin or use battery oprerated devices like radio triggers, and where optical triggers depend on flash to provide the power. Maybe Canon Speedlites has extra low trigger voltage, or too little current flows between the pins.
The only optical trigger I have that can consistantly fire and recycle my 580 and 430 EX is the Wein Peanut (PN), unfortunatly I don't have any other stand along optical trigger to do some crazy re-wiring/power supply type of tests. However, there are some odd behavior of the Peanut I observed that leads me to believe it has a different design than other triggers, relating to Canon EXs:
1) If I put the master 580 on a hot shoe adapter (thus everyting manual control), and connect to Peanut, when I fire another triggering external flash, the 580 will fire, but the slave 430 will not. But with the hot shoe adapter connected to the camera's sync port or the Ebay trigger, both flash will fire. Even though the Peanut can trigger each flash individually well, but it does not act as simple as shorting the center pin in master/slave mode.
2) In a different setup: master/slave--580/430 in manual mode, trigger by a sync cable. If I add a third flash into the mix that is triggered by the Peanut, lets say a Sunpak 383, the 430 would go nuts--it would either fire as the modeling light function is turn on (continue flash for 2 seconds) or not fire at all. Seems to me the Peanut is so sensitive that it catches the commanding signal from the 580 then the actual firing, thus confuses the heck out of the 430. Now if I take the peanut out of the equation, I could add a Alien Bees and/or another portable flash with built in optical slave, every light would work nicely together.
Is there a electrical engineer in the house?