For that many, you might want to start with an Open Table private dining search, since that will show you what restaurants can even accommodate that many. From there, we can probably advise on the best options. http://www.opentable.com/info/banquet...
A lot of them give TOTAL capacity, which means you're goign to have to rent out the WHOLE restaurant, but there are some good options, like Art and Soul, that can seat 100 as a sit-down.
if I were hosting a dinner for 100 people I would avoid the restaurant and start talking with the catering wing of a hotel in the vicinity where you want to be. You can rent a ball room and work with the catering director on a menu for the evening. another option would be to go to an area country club and see what they can do for you
any specific type of food or setting? dietary requirements? location requirements (i.e. accessible)?
We had our Friday sabbath dinner at Darlington House and it was fantastic - it wasn't 100 people but they have a good bit of private space that could. The service was great and so was the food.
Our "rehearsal" dinner was at Firefly. I really like it there but personally wasn't impressed with the pre-set options and all that.
I worked with CoCo Sala on wedding/rehearsal dinner options before it was nixed for a rehearsal because it wasn't walkable from the wedding hotel...but they were very helpful and the menu looked great. It money isn't an issue, there are places like the W hotel roof top that can rent out (which would be beautiful in May, though their service was horribly rude when I checked them out). Busboys and Poets on U Street also has a private room but I'm not sure of the capacity - we actually considered that for our wedding, and they were great to talk to/work with.
My wedding wasn't even 100 people, though...heh