Hi,
D3 - 12 MP
D4 - 16 MP
Keep in mind that D# series are intended for sports and low light.
The X-series is for studio/landscape type of users who shoot low ISO
D3X - 24 MP
D4X - 36 MP
Todays 16 MP APS-C sensors (Pentax K5, Nikon D7000, Sony SLT A55 etc) correspond exactly to 36 MP when scaled up to full frame, so the technology is here and kicking. My guess is that todays 16 MP cameras are not very far from the Nikon D3X, Sony Alpha 900 and Canon 5DII. I have tested my stuff (Sony) and that is certainly the case.
It is pretty sure that we will see a Nikon D800 with 36 MP very soon. Going from 24 MP to 36 MP is a minor step, that probably will not be visible in prints.
I would not call it a mega pixel race, but there is an evolution going on on sensors and one of the effects of that is more pixels.
It's exactly as with film. We had different directions, films intended for high ISO (Tri-X) and for high resolution (Panatonix-X, T-MAX 100, Technical Pan).
Best regards
Erik
Looks like the MP race is over.
http://www.engadget.com/2012/01/05/nikon-d4-dslr-camera/