Just went for my husband's N-400 Interview at the Washington, DC office and figured I would share with all of you, since we also have a pending I-751.
I went with him and they didn't ask any questions or even acknowledge me in the waiting room - they called him in, and then he came back a couple seconds later and told me to come along too. Did all the N-400 things first - civics tests, reading, writing, going over the document (I was in the room) and then did a quick overview of our marriage, and we were on our way! The IO told us that we would get a letter in about 2 weeks with the details for the oath ceremony, and that it isn't same day because of the I-751 - he has to adjudicate that one first, and then move on to the N-400. So I'm going to keep an eye on the USCIS website and see when that changes over, and we'll keep an eye out for a letter in the mail - glad it's almost over!!