A gx2 version of mobile parts like they did generations ago would still have only the raw sp power of an original 260gtx, just with dx10.1(which shouldn't be undervalued) but will be expensive, a dual core card and a total waste of cash.
They wouldn't go with the same spec as the mobile part, bus and SPs would have to be ramped up - but then they would run into the same issues as with GT300 with power leakage, yeilds, etc...
So looks like nVidia have backed themselves into a nasty corner...
TBH tho for atleast the next 6 months and probably 12 month there doesn't look like being any game releases that use DX10.1 or 11 - so they'd be better off in the short term concentrating on kicking out as many 55nm + high SP dual GPU cards as they can as cheaply as they can until they get to grips with the GT300.