There is often a misconception among non-Japanese speakers as to the weight that the title "sensei" is actually given in Japan. Basically, anybody who teaches anything is called "sensei", even if you teach bartending or (at an even lower level) English conversation. This would include workers at day-care centers and your kid's soccer coach. It's a term of respect (in that it's an honorific), but it doesn't carry nearly the weight that "doctor" or "professor" would in the west (although doctors and professors are also called "sensei"). Under this system anyone who teaches an Aikido class, even if they are a 5th kyu would be called sensei, and not many people would think twice about it. Of course, not many people would attach much weight to it either :-).
If someone's well respected you might call them "sensei" all the time, even cross-discipline, or you might not, but there's nothing magic about it -- it's just normal politeness, the same way that you might call Tom Hank "Mr. Hanks" and not "Hey, Tommy" if you've just met him.
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Additional note from J. Akiyama:
Unlike the English language, the Japanese language places their titles after the person's name. So, "Smith sensei" is the correct way of using this title, not "Sensei smith."
Additionally, it's not very correct to call oneself "Sensei." Whenever a teacher instroduces him- or herself to someone else in Japan, they'll just use their last name. They might refer to their profession as being a teacher ("What do you do?" "I'm a teacher"), but they wouldn't call refer to themselves as such. So frankly, introducing oneself as "I'm So-and-so sensei" sounds a weird and even a bit presumptuous to my ears...