tom:
First of all, if he wanted me to know he'd tell me. Secondly, it's none of my business. Last but not least, it's just NOT DONE !...IT'S PRIVATE !
haha, I didn't mean to start any squabbling

I agree that private things are none my of business, and that if someone wants me to know something private about them, I should wait until they want to tell me rather than ask.
But how a guy makes his living is nowhere near the same kind of 'private' as "what's your favorite sexual position" or something. I'm asking the kind of question that 95% of the time has a perfectly harmless, boring and innocent answer that nobody has to be ashamed of. People seem worried about the other 5% who might be criminals and want to keep illegal earnings under wraps. Gosh, I hope I don't seem like I have bad manners to a classy drug dealer or bookie, lol. But seriously, I don't think the original poster was asking because he planned on phoning the IRS about B.I. =)
I know I'm going on forever about this, and my goal is not to argue about it, the general topic of what's good/bad manners is something I always found interesting. There's a lot of stuff that is rude now but later won't be, and vice versa. It's kind of fun to think about the origin of some of these things, like... why was it rude to eat with your elbows on the dinner table?
Maybe the whole "where do you get your money" taboo came from the days of prohibition where half the people on the street were breaking the law every day, and there were millions of illegal businesses being run by otherwise law-abiding people. Asking about their living might make them feel nervous, like you were looking to dime them to the cops... or you might even be asking a gangster and risking your life.
Then again maybe it came from the world of the very wealthy, where commenting on the cost of things or asking about the source of their money implied you cared about money, and that automatically put you lower on the social ladder (as one of the plebeians who had so little money that they actually had to think about it vs those who had so much they never do).