In similar fashion to the GDDR4 based 1GB 2900XTs that popped up soon after initial batches of reference GDDR3 offerings were released, it looks like several ATI partners are planning to use DDR4 memory on select Radeon HD 3870 X2 cards as well.
Fudzilla report that X2s should start appearing in around a few weeks from now, but specific details about how much memory will be implemented or how fast they plan to run it at hasn't yet been revealed.
A few ATI partners have confirmed that they are almost done with their GDDR4 R680 design. Radeon HD 3870 X2 cards are currently selling for around 400, but they are all based on slower DDR3 memory clocked at 1800MHz for the reference cards.
This card should be just a bit more expensive and it should launch in a few weeks, but we don't expect any significant speed jumps with the new DDR4 card.
The point is, ATI can do it and the partners want it, so they will definitely follow up with the new refreshed card.