wow, what a situation. Well, congratulations. I first want to say that you need to do what you need to do for your career. Your current program will find another resident, particularly if you are in even a semicompetitive field. So don't feel too terrible for them. It's not like you are bailing after 3-4 years of a surgical program or something...you're an intern.
The first thing you should do is check the house staff manual for your hospital and find out how much notice you have to give before you leave. Generally there is a contract to sign for your PGY2 year, and it comes out around April 1st or so. Definitely you don't want to sign that. If I were you, I'd wait at least a couple more months before you tell your program. You don't want to risk them getting hacked off and doing something bad to you...it honestly probably wouldn't happen, but it's possible. You don't want your evaluations to be affected negatively. Therefore, unless you think the fact that you are leaving will "leak" out, I'd suggest waiting until Feb. or March or so to tell your current program. If they know you are leaving, you may also get less teaching/attention from some faculty, and that would be bad.
At the same time, you want to give your program time to find another resident. So it would be nice if you let them know maybe a bit ahead of the time you are absolutely legally required to tell them. Part of this depends on how good a relationship you have with your current program and PD...I personally wouldn't tell right now, as intern year has barely begun and they may get ticked off that you already plan to bail out on them.
If you are in a surgical field, then eager interns waiting in some prelim spot somewhere else to take a PGY2 spot will be a dime a dozen, and they will find someone good. If you leave a categorical fp or IM spot or something, then it's harder for the program because they have to find a new resident who can supervise interns, take over your clinic, etc.
