Depends on your employer. Do you have co-workers who have MBA and maybe were in your shoes at one point?
For me, my boss knows. I have to tell them, since we have yearly career discussions. Also, if we go out for drinks, he's going to ask me what I do in my free time. Since I'm mostly studying, I don't really want to lie to the guy do I?
It may have hurt my chances for a promotion. Maybe you want to tell him you are looking to continue working and go part-time?
Also, your boss will probably have to write you a letter of reco. He will have to know at some point right? If that is the case, then it's probably better to be upfront rather than drop the bomb and say I need a letter of reco for MBA. That will really take them off-guard.
I guess it just depends on your situation and employer. I work for a fortune 50 company and MBA is fairly desirable if you want to move up. I just want to get it done with before I get married, have kids, etc.