My house mates boss came over late at night once, he thought it mattered not that I have cream carpet on the stairs and landing and proceeded to track mud all over them... needless to say I was less then pleased as I like to keep my carpet clean, I told my housemate he would be the one cleaning the mess up... the next time he (the boss) came over it was shoes off in the hallway like a decent human being.

As a rule of thumb, if you walk in the hall way or kitchen (where I have wooden flooring) I don't mind if you keep your shoes on, but the lounge I generally make people take their shoes off, if you buy the most expensive underlay and a decent carpet you really shouldn't need to have your shoes on.

As for other peoples houses, it really depends on their rules and whether or not my shoes are dirty/wet, if the latter then I would always take my shoes off as I know how annoying it can be to get mud stains off of carpet.