I am sometimes in a rather interesting position, as I get out of my home dojo to train at some other places, in different associations. Sometimes (as this past Sunday), someone will refer to me as Sensei. I usually will ask them not to do that. Believe me, it is not because I am being falsly modest!
I think the instructor of the dojo should get that title (if it is going to be used at all). That is the person responsible for the training, the person who takes the risk of operating a place for training (financial and otherwise), the person that the members should be looking to for guidence in training. I don't mind when people ask for suggestions, or trying to help where people feel I can. But I like it to be clear who is teaching, who is taking the risk on behalf of their students, and who is ultimately responsible. I feel that letting people call me Sensei kind of get's in the way of those priorities.
And it's just less confusing for everyone. And as Jen said, sometimes we do just need to lighten up!
I don't use sempai much, since from what people like Peter have said, it seems to be something most properly used in a truly Japanese context. And I can't say I like everything that I've heard about it in that context...