My understanding is that employers may not hinder employees in discussing salaries among themselves, but may impose restrictions on providing salary information to non-employees.

So I can tell another Googler my salary but I can't tell you (unless you're a Googler) because my NDA forbids it. I'm pretty certain that I'm allowed to tell you that I'm not allowed to tell you, though. :)

There's an internal site where a number of Googlers share their compensation, anonymously or otherwise. I've never received any indication of disapproval or retaliation from management for being on it.

(I am not a lawyer, standard disclaimer applies.)