My income is known by only me, the tax guy, and God.

This question is usually indicative of (to put it mildly) a very shallow person. Someone who judges another based on their paycheck- obvious deadbeats and bums aside- is pitiable.

I think if someone wants to volunteer this information, it's okay, but to ask, I feel this is rude.

yes that is rude. But there are some people out there who offer it themselves so they can boast about how great they are doing. Just as rude. Personally, I do not care what someone makes. none of my business. would not ask.

Ask it of me, unless you're a prospective client inquiring what my fee is, and you're likely to get the classically rude "piss off" in return.

As a hiring manager I'll do it.  I suppose a reporter doing a story on salary equity can do it.  Otherwise--steer clear, this is seriously rude and MYOB stuff.

Years ago, a buddys wife insisted on knowing how much I made. They were having a party and the more she drank, the more she asked. When I told her I never discuss my finances with anyone other than my wife, she just went off the deep end. I had to leave, it was a pretty embarrassing scene. Pretty rude question, especially if you already made it clear you don't want to talk about it.

Even though some kinds of salaries are public knowledge and easy to track, I still think it's rude to ask this question. Everyone deserves a certain amount of privacy!