Gopher, forgive me if you already addressed this, but where are you applying? There are a number of great programs within a few hours of DC (DC, Baltimore, Philadelphia, Princeton, even NYC, etc) and my quick scan of the thread seemed to indicate that your SO would probably not have more than a year or so out of the country. You might be able to manipulate the situation so that you are long distance during the week but together each weekend.
With all due respect to flyers29, I think that you should tread very, very carefully if you plan to ask her to hold off on an FSO career until you finish. To my ear, not only does it sound selfish, it is extremely selfish. After all, a lot can change in 5 years- maybe you have a few kids in grad school, she needs to find some other job, etc and the FSO job becomes no longer feasible. If you do propose that she defer, I would also suggest thinking about whether you can put off school for a year and then try to find somewhere close to her in DC. Both grad school and long distance is hard enough that you don't want to start out with resentment.
Good luck. I've been doing both long distance and multi country for almost two years now, and it really sucks.