Tricky subject this. My understanding is that shihan is used for the originator of an organisation or system and, in judo at least, we do not have many of these. When being instructed by someone we perceive as shihan we address them as sensei or family name + sensei but refer to them in the third person as shihan (with family name preceding.
Sensei denotes experience and a degree of technical ability in almost any field - doctor, teacher, artist, engineer eceteras - my understanding is that the meaning is something like 'one who has been there, seen it and done it' and not just purchased the 'T' shirt. Age and grey hair may have something to do with it.
Although we generally refer to and address the instructor and other high grades as sensei while practicing, the technical advisor of the British Judo Council, Hosaka Akinori (family first, personal name follows!) sensei, 8th dan kodokan, dislikes us using the term sensei when refering to ourselves or our fellow dan grades. I think this is because he sees all this senseing one another as an unseemly ego trip. He could well be right.