Old thread, but I found it doing a google search trying to answer a similar question as the original poster.
Just found out, or understood, that in our system no one is a sensei until they are 3rd dan or higher.
2nd dan and lower are sempai.
In the same token, a brown belt can teach here - seems I read somewhere that stateside usually 2nd dans teach.
People here dont typically get their brown belt, from what I have seen, until 7 years or so.
Black is typically longer by the looks of it.
Anyway, it seems it really is a depends on system then.
I was under the impression that if you were black belt and teacher you were a sensei.
The Shodokan system is similar, if I'm remembering correctly. 3rd dan are referred to as sensei, 2nd dan are not. I was training briefly under a pair of great 2nd dans and when one was promoted I was told it's more appropriate to call him sensei now.