To clarify, Metro to Dulles will NOT open in 2013. Only the first phase of the project, to Wiehle Avenue (just past Tysons Corner), is expected to be finished in late 2013, and that is if nothing unexpected impedes progress. Phase 2 is only now completing preliminary engineering, and if all funding comes through and all goes well with construction would not open till 2016 at the earliest.
Car service or taxi is the easiest and fastest, but for those on a budget the options would be
Metro Red Line to Metro Center -> Metro Orange Line to West Falls Church -> Washington Flyer shuttle bus (
http://www.washfly.com/ ) $2.75 + $10
Metro Red Line to Metro Center -> Metro Orange or Blue Line to Rosslyn -> Metrobus 5A (
http://www.wmata.com/bus/timetables/dc/05a.pdf ) $1.60 + $6
(absolute cheapest but also far and away the longest)
Georgetown Circulator to M St & Wisconsin Ave NW -> Rosslyn Circulator to Rosslyn (or walk about a mile) -> Metrobus 5A $1 + $6
While it is true that weekend track work can add significant delays, it is usually announced ahead of time on th WMATA website. One thing to note: the most disruptive shutdowns are scheduled for long weekends— holiday tourist crowds being nonetheless smaller than weekday rush hour crowds— so be extra wary of Metro around public holidays.