I think what you're describing is found in the Paycheck Fairness Act. Here is a discussion on it, and I do noth think it is law yet (but I haven't checked):
http://www.nwlc.org/pdf/PaycheckFairnessFeb2002.pdf
Now, I've worked under this gag rule regarding salary a LOT. But, one can always discuss and issue without "discussing the issue," no? This Act you mentioned is targeted at the wage gap women experience. That said, it has at least one benefit that crosses gender lines: it prohibits wage rate gag orders.
That said, it strikes me that reality triumphs. Most employment is at-will, I know mine is. Even when Will's not around that day.
Seriously, though, if they want to fire you for one thing, but can't, they'll find another.
So, do what lawyers do: start internet websites like "Greedy Associates" where we go to anonymously compare this stuff and spread muck about the big law firms.
