As always, great forum. Everything that has been mentioned about 580EXII being optical only, STE3 being radio only, and the 600 being both radio/optical is correct. I would only add to this, there can be more than 1 Master. We tried this, this past weekend at a wedding ceremony, at the beach and at night, so it was a real nightmare as far as lighting goes. We clamped 2 600s on the sides of the huppah (1 on each side) and acting a slaves. My wife was covering the ceremony with 2 5D3s, one with a 24-70 and the other with a 70-200, each camera with a 600 acting as master.
This worked flawlessly, she would switch from one camera to the other, and the slaves would fire without issues. This is properly documented on page 60 of the manual, if you don't exceed the limit of 16 units in total (slaves+masters).
Canon's radio system works great and it is super dependable. In my experience, it works 1000 better than pocketwizard, which was my previous solution. And the fact that the Master lets you know that all the slaves are ready is awesome.
I am selling my remaining 580s to replace them with 600s.
Happy shooting to all.