DC United just announced its fixture list (link:
http://web.mlsnet.com/schedule/season.jsp?team=t103) - and of their 15 regular home matches in MLS, seven go up directly against Nationals home matches.
The conflict days:
Saturday 26th April - Nats vs Cubs (19.10), United vs Real Salt Lake (19.30)
Saturday 24th May - Nats vs Brewers (19.10), United vs Toronto FC (19.30)
Wednesday 4th June - Nats vs Cardinals (19.10), United vs Houston (19.30)
Sunday 22nd June - Nats vs Rangers (13.35), United vs San Jose (15.00)
Sunday 29th June - United vs LA and David Beckham (12.00), Nats vs Orioles (13.35)
Saturday 2nd August - Nats vs Reds (19.10), United vs Kansas City (19.30)
Saturday 30th August - Nats vs Braves (19.10), United vs Red Bull New York (19.30)
Fortunately on two of those days (the fourth and fifth pairings above) there's enough of a difference in start times that I'll be able to see at least parts of both matches...but between the Nats, DC United, the Mystics, the Redskins (schedule normally comes out in April) and possible playoffs for the Capitals and Wizards, I'm going to have some real schedule resolution to do. Normally I opt in favour of whichever teams play fewer matches, but this time I don't think it'll be that simple (too many Nats matches missed!).
But that's just my Six-Pack life...what could turn into a real problem for a lot of Nats supporters is the parking situation, assuming the Nats shuttle from RFK becomes a reality. DC United season ticket holders (full- and half-season) get discounted parking at RFK, to say nothing of the non-STH fans who combine to produce attendance in about the 17,000-22,000 range (it'll probably be a lot more when LA is visiting - New York is also a big draw)...combine that with the Nats (especially against a big draw like the Cubs or the Orioles) and it could be a real mess.