Now, as of several weeks ago, there is a new place called Pitango. It's located on P Street, N.W., between 14th and 15th Streets. I tried a few flavors there a few days ago, and the experience was pretty amazing.
I will (of course) be back to try other flavors, but I thought that both the Cinnamon and Bourbon Vanilla were excellent. They also offer several flavors of sorbet.