Yet if you ever ask anyone what their pay has EVER been you're being rude.

I guess that's the part that gets me. You can't even ask someone what they used to make in a different position. If it's not your current salary or even your current job, why does it have to stay a secret?

As for companies who demand that applicants reveal their salary requirements, I'd be very suspect of their motives unless they explain upfront that a salary offer is contingent upon the potential value the person brings, as well as the projected salary range. They shouldn't reject someone whose demands exceed that range, but rather leave it up to the individual whether he or she wants to stay in or withdraw from consideration.