The rooftop at POV/W Hotel is pretty amazing. Not sure what the pricing goes for a private event up there but it would be a really nice event.
Marvin on U Street also has a nice rooftop and I know they will let you rent it out for a private party.
U Street Cafe
1301 U St NW, Washington, DC 20009
While I think Marvin's rooftop is a great place to spend a Sunday afternoon, I'd definitely describe it as rustic/casual. They have great food, though, and a long bar with a space that's great for mingling.
Also, is anyone but me pissed that the neighborhood nixed Churchkey's patio?
Tabaq has an awesome upstairs patio space with a glass roof that retracts in nice weather - the view is great, also, although it's the view over the neighborhoods north of U St, not the mall. It feels very authentic DC to me. I know they rent out the whole space as a couple was in there asking about it the last time I was there. Really nice ambiance, but also very laid back and unpretentious-feeling. The drinks we had were great.
Naturally, you could go with POV, but I bet they cost twice as much. Still, I know they reserve out the front area on the other side of the bar for private parties, and I'm sure that's more cost effective, but I don't think the service is very good there, and you're still sharing the bar with the rest of the lounge.