IMO a die shrink to 55nm could enable the posibility of doing a GX2. Maybe not a GTX280 GX2 but yes one with slightly lower clocks or one cluster dissabled and with enough performance to crunch the X2, of course it would require more power, but it would still be within the 6+8 pin envelop. I have three "facts", but of course are only based on my opinion:
1- You have to take into account how power consumtion works. It's exponential, not linear, so a slower part would consume a lot less and the same can be applied to voltages. Nvidia because GT200 was worse than expected in this area had to lower the clocks, but probably they have kept it as high as possible within the selected power envelope. There's always a hot spot for performance-per-watt for any chip and GTX 280 is probably quite higher than that spot. FACT: look at Wizzard's Zotac AMP! GTX280 it consumes a lot more than what you should expect from that overclock. Aim a bit lower than that said spot and you have a "low power" chip. For example a GTX280 GX2 @ 500 Mhz would consume a lot less and still leave the HD4870 X2 behind in performance.
2- Nvidia has implemented the abbility to shut down parts of the chip in the GT200, and it really works very well. Again look at Wizzard's power consumption charts and how it consumes a lot less than the X2 on average, even though its maximum is almost the same. That would make the card to probably never reach the maximum power consumtion in the GX2 card. There's no way you are going to be able to make a total of 64 ROPs work at the same time, for example.
3- Continuing with the above argument, IMO if Nvidia did a GX2 it wouldn't be based on the GTX 280, but on the 8800 GS substitute. Nvidia will surely make a 16-20-24 ROP card while mantaining a high shader count (maybe 192/168 SP, same or one less cluster than GTX260 for example), they would be stupid if they didn't, as it makes more sense than ever. The GS is "weak" because it has 12 ROPs but 16 on the other hand are enough for high-def gaming. 16 ROP x 2 is more than enough as the X2/GX2 can testify, 32 x 2 is just over-over-overkill and silly.
My bet is that Nvidia will do a 20 ROP 168/192 SP card for high mainstream no matter what and they could use that for the GX2. Final specs for that hypothetical GX2 would be: 40 ROP, 336/384 SP, 112/128 TMU and 2 x 320 bit memory controler, that if they can't make the card use the same memory pool as R700 seems to be going to do. The above card would leave the X2 well behind performance wise and still be within the power envelop IMO. Of course that envelop would be higher than that on the X2 but reachable IMHO and still within the 6+8 pin layout that's 300W, the GTX 280 needs 6+8 pins just by a hair.