Check out this article by Yamada Sensei. He wrote it in response to the recent promotions of some instructors to shihan. For a full listing of the USAF-ER shihan go here
Rules Regarding Shihan -- Yamada Sensei
Note in particular the following:
Now let me explain what Shihan means in my culture. Please understand that I am not trying to make Shihan seem unimportant, but I must express it in the terms that the Japanese people use the word and what it actually means to us. Shihan is simply another way of saying Sensei. As a matter of fact, it says in the Japanese dictionary that Shihan is used to call the teacher in a particular field or profession such as martial artist or cultural leader. For instance, you call a school teacher in Japan Sensei and you call a teacher of martial arts Shihan. When I make an application for an official document it's proper for me to write Aikido Shihan rather than Aikido Sensei.
I don't know when exactly my students started calling me Shihan. There were no restrictions or regulations. My students simply started calling me Shihan as I got older and more respected. If my students called me Shihan for example, when I was around 30 years old and 4th dan, I'd probably tell my students please don't call me Shihan. But the title of Sensei and Shihan should both come from respect that the instructor earns from their students and the Aikido community.
Please remember, my intention is not to put down the title of Shihan, but to explain how it is used in the culture it comes from.
As for myself, I still prefer for my students to call me Yamada Sensei. It feels more friendly and comfortable to me.
And we still just call him "Sensei." The only times I have seen "shihan" next to his name is on flyers advertising a seminar