Actually nobody here can answer that question for sure unless they have some insider information on the nature of the 4870X2's new interconnect. I suspect that it will work, but it probably won't be ideal.
I also suspect a single 4870X2 will be so blindingly fast that there will be no need for another 4870.
Yep, it's hard to know how much that CrossFire Sideport will affect. Maybe it's like three normal 4870's together, without using that bridge in those situations? In anyway if 4870x2 is "blindingly fast" it does not mean that very soon we will not need more speed... :-)
There are allways situations when more speed is better... How about crysis with AA on at very high settings... maybe we have to wait next generation for that... or special four core version of the 4870x4 ;-)
Easy solution, external craphic card with four cores glued together with those new CrossFire Sideport's... well maybe not so easy solution after all... It's better to wait ATI 5000 series and Nvidia 300 series for real Crysis support... They really put hardware down to their knees with their engine this time.