Dear money9111,
It is a very precarious conversation indeed and that is why many people often fear asking a current boss will put their job in jeopardy. So, as long as you believe you have a good enough relationship with your boss that will 1) result in a great, glowing recommendation and 2) will not put your job in jeopardy, then you definitely want to ask your current boss to support you in your quest for business school. There are several approaches you can take to the conversation. Some folks choose to bring it up during annual performance review time as being another step in their professional development. Another avenue might be in the context of a mentoring conversation where you ask your boss for his/her advice on what you need to do to advance to the next step in your career--then you can skillfully bring up an MBA if your boss does not. If you have a social relationship with your boss, you can also invite your boss out for a drink after work or out to lunch where you can bring up your desire to apply for business school.
With regard to the possibility that you will be gone in a year, that is a real possibility and one that your boss will have to deal with in the event you are admitted to a school you decide to attend. However, your boss will have the advantage of knowing several months in advance, as opposed to two weeks, if he/she is included in the process. Including these types of points in the conversation might also help to calm any anxiety you sense on the part of your boss on the thought of losing a valued employee. Conversely, you also want to emphasize your commitment to the company while you are still an employee--that your performance will not be affected by your business school application process at all. Hope that helps.
Good luck,
Lisa