The truth is that until we take delivery of a few of these, and can do some side by side comparative shots of the D800 to the D700 we aren't really going to know whether 100-6400 ISO rating is truly equivalent.
I am hoping that it is roughly equivalent. Then, you have all of the other advantages of this camera. As a wedding photographer I would like that it would be advantageous to be able to shoot video as part of your package that you sell to your clients. I would also assume that many of the settings during a complete shoot would include an outdoor reception, times when you can use flash, as well as the church setting where you must rely on high ISO.
I think this is also a case when that VR lens is really going to come in handy. I can see the 70-200/2.6 II with active VR being a huge advantage in that church and with all those megapixels. I think the capabilities of this camera will cause you to favor those VR lenses.