Syl Arena just did a lecture for Canon CPS on the 600 where he makes the strong point that it is not an infrared system
I like Syl. He's a good guy and has great advice for using Speedlites. But about this, he's wrong. As I noted above, with a Speedlite as a master, the main tube transmits the signal, so both visible light and IR are being emitted - but the receiver is only picking up the IR. As I also noted above, the ST-E2 outputs IR only, since the flash tube is covered with the same visible light blocking IR-transparent filter material.
Don't believe me? Try reading Canon's own specs
for the ST-E2...
Transmission System Infrared pulse
How is that not
Syl states that for Canon's optical system, the transmitter is the flash tube (which I stated), and that the ST-E2 uses a flash tube as well, but it has a "thick piece of plastic over it" so it's not "throwing on-camera flash into your shot." If that 'thick plastic' blocks the light from the flash tube, how is the ST-E2 is signaling the slave flashes? Infrared. You can't see it. Your CMOS sensor can't see it. But the slave flashes can. Syl just seems unaware of the fact that a flash tube outputs a broader spectrum than just visible light, and Canon is using the non-visible part of that output for optical wireless triggering.
The real point he's trying to make is that the IR triggering is 'line of sight' just like visible light. A TV remote control is IR, and it's line of sight, too.