Try to keep your mouth shut until you have a job offer, especially if your move is not entirely certain. There are only a few cases in which I think it would be appropriate to tell your boss what's going on. For example, if your spouse is being forced to relocate, obviously you are going to go, and if you have a good relationship with your boss, then it might take some stress off of you to tell the truth. The general rule, though, is not to give your employer more power over your destiny than you have yourself.