I didn't read the thread but I assume someone said that the answer depends on the relationship you have with your boss and his/her personality. I agree with that person.
There are a lot of open job positions out there, but there are way too many people who either can't/won't look outside their field, won't accept a pay cut, or who are happy to mooch un-employment as long as they can.
In that case, maybe you should go and tell him and tell him that, so he won't feel the need to keep you by giving a bad reference, since you'll help him replace you, and thus he'll be cool with it.
Well, thinking about my relationship with him, he'd probably be pretty cool about it regardless, but I would include that when it comes up. I honestly could train someone to do what I do in a few hours anyway. A lot of it was otj training