It does not fit, the new RRS plate will have to be made the new 1DX body as the fron of its vertical grip area now with a little contour while the 1DsIII is stright. I have a pre-ordered RRS plate not coming yet, and I was in fact waiting for the Capture One Pro to finally support tethered shooting for either 5DIII or 1DX so I will sell my 1DsIII + RRS plate, now I am just waiting. The 1DX is really nice, I received mine last weekend in Bangkok, not really tried it yet.
Actually the L plate from a 1DS MK3 DOES
fit the 1DX. It just doesn't fit perfectly - but its certainly useable.
I have my 1DSMK3 with a RRS L bracket here next to me and just test fitted the bracket to the 1DX and it bolts on without problem. Its not a perfect fit, but its easily useable and can be used without major issue. See attached photo. (sorry for quality, just snapped it on an iPhone)
BTW: You can also use a tripod collar plate from a 70-200mm sideways as an interim solution until RRS ship the 1DX L Bracket. Of course this doesn't give you a vertical mount, but it does at least give you a horizontal mount (if like me you want to keep your 1DS MK3 bracket on the 1DS).
I have also ordered the 1DX L Bracket - but as above, in the short term you can certainly use the 1DS MK3 bracket.
EDit - forgot to add. I always offset my bracket as shown in the picture so that I can use a cable release vertically.