Only Verizon and US cellular.
EDIT: you can enable it if you're rooted by editing a file in the system/csc folder.
I'm thinking that the part in bold could be your snag.
Seriously though, some carriers are enabling this and some are not. A serious flaw as universal availability of the feature would make it infinitely more effective.
You can enable it yourself by availing yourself to a custom ROM or by editing the properties XML. Once enabled, the menu shows up under Settings > General > Security
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