I think for the time being, until the D800/D800e is in the hands of capable photographers, especially those that have the opportunity to shoot and compare files taken with the D800e along side such cameras such as the 645D, S2 and/or various other near 40MP MF cameras, it is an exercise in fruition to determine which system has the advantage. Of course among other things, lens selection will play a considerable role in these comparisons as will the intended application. There are just too many variables. One camera may actually end up with showing better acuity in details at low ISO or maybe lose that advantage at higher ISO. One camera or another may have more color depth or dimensionality or represent some subjects with more realism or better color and/or skin tones. We're also of course comparing different sensors (CMOS vs. CCD) and electronics that relate to output. It may be that with certain lenses and settings there is little difference on web viewed images or smaller prints but substantial differences can be seen in large format prints. I know what I observed with the early posted samples and what I liked or didn't like, but I personally will reserve judgement until better and more meaningful comparisons are made.
I've learned a long time ago that numbers alone rarely tell the story when it comes to final image product.
As for portability and other non image related factors, Tom is correct in that many of the AF lenses (from wide angle to 300/400mm) in the Pentax 645 system (for example), may be similar in size to their full frame 35mm Af counterparts for transport consideration, but relative size of lenses and even body size has little consequence unless the system when used with those lenses is the best system chosen for a particular application. If I am shooting fast moving wildlife or pro sports, I'd choose the D800 (even though not a speed demon compared to a D700/D3/D4, some of which I use) every time with appropriate lenses (regardless how big) and yet conversely I would choose my 645D every time for landscape and portrait and macro work, if it turns out that the qualities of the file (as I outlined) continue to stand out and be superior to Nikon DSLR's as it has been up to the present (D800e excluded of course for the time being). It stands to reason that any subject can be tackled with any camera (I've shot some captured wildlife with the 645D), but having the right tool in order to obtain files that achieve a particuar goal in capturing an image, as envisioned, is personally my main consideration and why sometimes I will reach for a D3 or D700, where at other times the 645D is the right choice.
Once the D800e is out there, then let the comparisons (and fireworks) begin, especially that there are so many other tangible and intangible reasons why one will still shoot with one system or another as their main system.
Dave (D&A)