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.)

This is definitely untrue. I've worked at Google, and salaries were shared internally very much in the open. The official response was something like "we discourage sharing your salary because we think it's bad for the working atmosphere, but we won't stop you". I shared my salary publicly and there wasn't any form of retaliation.

and seemingly not true?

http://qz.com/458615/theres-reportedly-a-big-secret-spreadsh...