Sony already HAS MADE a 24.6 MP full-frame sensor: it was used in the Sony in the A850 and A900 bodies, and is currently still in the Nikon D3x. Interestingly, the noise and overall color response,dynamic range, saturation,and resolution of the SAME "sensel" (the light-sensitive part that SOny made and sold to Nikon for the D3x) was quite a bit different between the Sony cameras and the Nikon D3x. Nikon spent a ton of engineering, design, and manufacturing effort to improve the electronics and probably the entire filter array of the D3x--and the camera's $7995 price tag allowed them to do that. Sony, OTOH, sold the A900 at a really LOW price point, and the A850 even lower, at $1899 on the A850, and around $2200 or so (depending on region,time of lifespan,etc). The Sony A900's high-ISO performance and color responses were not the same as those of the D3x; the D3x did a lot better at higher ISO's, but the final cost of the D3x has made it a pretty rare beast.
We've gotta remember: Sony has already gained the lead over Canon on mid-level sensors and noise performance: the Canon 7D's sensor is well,well beneath the Sony sensor the Pentax K5 and Nikon D7000 are using. Canon cut down the MP count in its G10 from 14.7 to a 10 MP sensor in the G11; for Canon, the 18.2 MP sensor, 17.8 or so effective in the 7D has issues with color depth and dynamic range, and it seems that they need to TOTALLY RE-WORK the electronics...which is what Nikon learned going on five years ago now; the sensel, the light-sensitive part that they buy from Sony isn't a complete sensor until Nikon pairs it up with read electronic, Noise Reduction capability,Nikon's proprietary color matrix demosaic routines, and the AA filter array...the sensor is part of the equation---but one of the real KEYS people keep overlooking is how LOW READ NOISE electronics can take a fine, basic sensor, and then with the right camera-maker skills, the performance can be elevated to superb CAMERA performance.
We need to step back just a bit and look at the final "camera" performance, not the sensor specifications!!!! The A900 and D3x comparisons showed this conclusively. Same with the Canon 7D versus the Pentax K5 and Nikon D7000. I have a feeling that, just maybe, the "36 megapixel" specification is indeed true. But----and this is the biggie---what Nikon does with that data is going to be the result of NIKON engineering, not Sony engineering. Nikon might do what they did with the older D1-series cameras--those were called "2.7 megapixel" cameras, but Nikon revealed years later they were using a 10-million photosite sensor, and were in effect "binning" the data...using basically three data points to create one data point. This is a good way to reduce noise. It's probably more cost-effective to do that that other approaches. I think with a 36 MP pixel count, Nikon's engineering could figure out a way to apply Noise Reduction, as needed, to produce very good, usable files, EVEN AT HIGH ISO values...
This ain't 1999, or even 2006, any longer...the old idea that too many MP automatically lead to "objectionable noise"...well, that idea has been made one heck of a lot less valid with the advances in noise reduction through better electronics. A camera is not just a sensor--the camera part of the equation is critical. Nikon, and Pentax, are now using Sony sensors that can capture an image with the exposure set wayyyyyyy too brief (as in say, proper for ISO 50,000), and then the sensor exposed with an exposure properly set for ISO 200, and then the resulting BLACK raw file can be opened up, and a quite decent image made out of the data. I have seen the demonstrations of this from the Pentax K-5...ISO 50,000 at gain setting of 200 in-camera, then the file rescued....the results were what I would describe as "miraculous".
I have not seen anything even comparable with any other sensors...so...whatever the new Sony sensors bring, and whatever the electronics, demosaic routines, and image processing Nikon adds to the equation, I really do think the new "D800" will make good images. If it has a higher number than the 700, the core capabilities will be higher and better; that is the way Nikon iterates products. An updated product gets a letter added....an entirely new level of core capabilities gets an entirely new model number. Sorry for the length of the post, but just had to lay out my thoughts.