Henry

I beg to differ with the above posters. I wouldn't recommend using the metro after rush hour - that is, not after 7 PM. The metro itself is fairly safe, but the areas around most stops are not. After a long day of sightseeing (please stick only to the main sites - do not venture into DC neighborhoods), you will be tired anyway. Just stay in your hotel.

kayd

I'll disagree with Henry. Inside Metro, you are safe at any time. The areas around some stations can be a bit chancy, but those are not places a tourist would go anyway.
Since Laura plans to stay in Rosslyn, she can venture very safely via Metro to Pentagon City (much nicer than Ballston) for shopping and a movie or to Courthouse for a movie and dinner. For Georgetown -- movies (a new 14-screen multiplex), shopping, and dining -- a taxi is the quick and inexpensive answer if you don't want to walk the mile or so from Key Bridge to Wisconsin Ave.
Your hotel concierge should be able to provide a detailed street map of the immediate area and recommend restaurants you might like.

r-travels

I agree w/ Kayd and disagree with Henry too, and wondered why his position. Metro stops I would have, or have had, no problems (even up to 11 or so) with include L'Enfant, Gallery, Union Sta, Smithsonian, Arlington, Rosslyn, Pentagon City, King St, Crystal City, Foggy Bottom, Dupont, Cleveland, Courthouse, Clarendan (sp?) and Greenbelt. And those are just ones I can think of right off the top of my head. Sure, I'd avoid certain areas, but for the places most tourists would want to go, using the Metro should be absolutely fine.