Staff in several photography stores have been mistakenly saying that the Canon 600EX flash cannot be used as a master with 430EX slaves.
Wrong. It certainly can. Proof:
A 430EX acting as slave to the 600EX on my camera.
The 600EX has both radio-controlled remote abilities, and light-controlled abilities like the previous generation. You can choose which type to use: set it to light control and it’s just like a 580EX flash.
Rumour debunked, I hope.