Hey Andrew, check out the Potomac Polo School at the Red Eagle Ranch at http://www.redeagleranch.com/. I have never been there, but signed up to their updates on their yahoo group a while back and they seem like a great bunch of people. I think the guy that runs it is named Dante and a lady named Mary Muldoon own it.
Let me know how you get on if you end up contacting them.
Depending on what you are looking for I can suggest a couple places. The heavy hitters in VA, half an hour-45 min from DC would be Great Meadow Polo Club (http://www.greatmeadowpoloclub.com/) and the Virginia International Polo Club (http://vipoloclub.com/index_content.htm). Both offer all levels of polo and some great people to play with. Email me if you have a question I haven't answered or if I can help you any other way.