I would have bought a D3S for the low light performance, but switching all my lenses makes the cost of a body look like chump change.
Tha main advantage of high mp is the ability to crop, and I do a lot of cropping of my 5D MK II images with good results. If you fill the frame of a 12 mp camera the image will be excellent, since 6mp is about all you need for most prints.
In the last sentence lies the point as to why 12MP is not an issue for many shooters: an image file is not a final product and, in most cases, clients never get to see the image file. If you consider the portrait/wedding world, clients buy prints, mounted prints, albums etc. In many areas of photography (there are exceptions to this) the client does not care what camera you use, provided you deliver the results - which the client can hang on the wall or moor on the coffee table.
For press shooters, for instance, it is more important to get the shot (i.e. AF, high ISO capability, fps) than being able to blow the result up to billboard size.
(There are more specialised areas where the specific gear you use matters more.)
for my personal photos, I like to have as much resolution as possible - who knows what I may want to do with the image in 10 years time. For the material my wife shoots for her clients, the files will all be deleted after a few years anyhow - you can't keep all the material for ever, so as long as there is enough resolution there to produce the products the client wants, that is good enough.
Camera manufacturers have done a good job so far, continually convincing us that we always need more megapixels, in order to sell us new cameras. With the exception of a few users who really need high resolution, the times have changed, and Canon, Nikon et al need to find new reasons to convince us to shell out for new gear if our current gear isn't broken.
Digital has been wonderful for Canon and Nikon, because we have all been buying new bodies at a much higher rate. Film bodies had a much longer "lifespan" before anyone worried about upgrading, with the end result of lower sales. (Of course we didn't spend less money, because a lot of our money went into film and developing.)