Depends on the person and the context. My father is a (rather senior, at this point) history professor (w/ a Ph.D. of course) and his students and colleagues always call him Professor G. or Doctor G., but people who meet him in other contexts (outside of work) just call him Mr. G. or Steve.
Several friends of mine have Ph.D.'s but now work in completely different fields, and therefore don't really even think about their Ph.D's any more. They never use Doctor. (For example, I have a friend with a doctorate in literature who now works in internet sales and marketing -- it would be a bit absurd for him to insist on Dr. Y; in fact, it would embarrass him to be called that).
But I have met some people (strikes me as a bit pretentious) who insist on Dr. whenever they are introducing themselves, no matter what the context.
So, generally, I would say use "Dr." where professionally appropriate, and where it seems important to the person addressed (i.e., they brought it up with you!)