I know this has not been an option for Nvidia cards int he past, but I'm wondering if there's any chance of that changing with the GK104.  As the 690 is using the same gk104 from the 680 at a slightly lower clock speed, I'm hoping that becomes an option (I've pretty much convinced myself I want 3 gpus for my 3D Vision surround setup) however unlikely it may be.  Anyone heard anything that might give me some hope, or should I stop thinking like an AMD customer? ;)

Usually nVidia responds to AMD with a counterpart product or service, like Surround versus Eyefinity, but despite AMD being able to mesh multi-GPU with single-GPU cards for the past few years, nVidia hasn't bothered doing the same.  Therefore I'm more inclined to believe since it hasn't been on nVidia's top priorities, it might never.
 
Afterall, there has to be a large enough pre-existing demand before any company tries anything new, there is no "let's see what happens" in this industry.  Perhaps not enough try such a combination that AMD can maintain a monopoly.
 
I haven't look into the scaling of AMD Radeons pairing multi-GPU with single-GPU cards, I think I'll do that now.

I'm guessing the OP got their idea from here:
http://www.hardocp.com/article/2011/04/11/amd_radeon_69906970_crossfirex_trifire_review/
 
The 6990 uses an onboard PLX bridge chip to supply bandwidth to both GPU's on the 6990. Since this company seems to have been adopted by nVidia as well, there's at least a consistency on the hardware front.
 
However, sli and crossfire are different technologies and nvidia has yet to say anything about mixing and matching GPU's. In the early days of SLI, even mismatched firmware would sometimes cause rejection between paired GPU's. 
 
Personally I would say this is highly unlikely given that just like crossfire, sli requires that GPUs sync clock speeds. If you could pair a 690 with a 680, then the 680 runs at 690 speeds. I'm guessing nvidia has a stigma against running under reference speeds (you're, in a way, overcharging for performance) and would never allow it officially.
 

Highly unlikely,  ...  and still wondering after all these years as to  " why not ? "
 I thought by now Nvidia would have implemented this feature awhile back but sadly that hasn't happened. It's only obtainable to use different cards as a "PhysX only" scenario .....

tpb7463

Personally I would say this is highly unlikely given that just like crossfire, sli requires that GPUs sync clock speeds. If you could pair a 690 with a 680, then the 680 runs at 690 speeds. I'm guessing nvidia has a stigma against running under reference speeds (you're, in a way, overcharging for performance) and would never allow it officially.
 

 
This doesn't appear to be the case anymore with the dynamic clocking of the 680's.  I've seen threads discussing the fact that the 680's choose to run at different speeds in sli, so I don't think this would be a factor anymore.  Since the gtx690 and 680 are using the same gpu's, with the same amount of ram, and speed not mattering, I was hoping it might work, despite the fact that it was not mentioned at launch.  But, the fact that the cards will obviously have different firmware indicating different models would probably kill 3 way sli unless Nvidia decided to allow it in their drivers.

Yeah, clock speeds don't seem to matter anymore. I'm running sli and one card boost further than the other. One hits 1097, while the other bounces between 1084 and 1071. During games where the cards don't need as much power, they can be hundreds of MHz. apart. Though, I've usually noticed some stutter when this occurs. I'm not sure if it is the fact I am using adaptive vsync or if it is because the clocks are all over the place.
 
There is a lengthy thread on the nvidia forums about vsync issues with 680's.

Now that the NDA is over and EVGA has officially launched, can we get any word on this?