What would you do in this situation?

Here's the situation-I took a job back in March at a daycare center, part time as a floater (which I am very overqualified for but I needed the job). When I was hired I was told flat out that if something opened up I would get the position. Then she went and hired another full time teacher for the cl I wanted. This past summer, I was promised a full time position in December. One of my co-workers (the one I was supposed to replace) decided not to leave, and decided she wanted more hours. So over the last two weeks, I have lost 25 hours. I am currently putting in applications elsewhere, and my dilemma is this; if I get an interview and get hired for the position I want (which I should, because of all my experience and education) they want to hire ASAP. Should I warn my current boss that I am looking to leave? I don't want to put in my two weeks notice in case I don't get the position, but I don't want to walk in one day and say "I quit, buh-bye". Not that I would feel guilty after how much she has screwed me the last few months, but it would be unprofessional. What would you do?