The only thing that we've heard straight from a developer is that it needs 3GB of RAM minimum. The rest is up for speculation.
My personal guess is based on Luthier saying he snaps pics and videos on his own PC which is old, yet the videos we've seen are smooth with adequate frame rates on his chosen (i'm guessing low) detail settings and it still looks good enough.
Plus, we saw the videos from Igrommir where it ran with stutters, albeit on a below mimimum specs machine and at full detail.
Igrommir PCs were i5 with nVidia 460, but only had 2GB of RAM and that forced the game to buffer textures/read from the hard disk too often when flying over land, resulting in the stutters. Over the sea however it was smooth enough, so i'm guessing it was entirely due to the lack of additional RAM.
Taking all that into account to make an informed guess, for minimum requirements i'm guessing:
CPU: dual or quad core (most likely quad, unless the dual is running at least a specified high clock speed or more, eg it might run on a 3.2Ghz intel core2duo or equivalent AMD CPU, but not on a 2.6Ghz dual core)
RAM: 3GB as mentioned before
Graphics card: Something like Ati 48xx or nVidia 28x (ie, the previous, DX10 series of cards) for good FPS at medium to medium-high detail. On lowest detail probably even older cards might be possible (eg, nVidia 8800 if running low detail and DX9 only)
Hard Drive space: Anywhere between 10 and 20GB.
I think it will not have steep
minimum requirements to just run it acceptably if you are willing to turn off a few visual goodies, but it will take a real monster of a PC to max it out completely.
When IL2 was released there was almost no PC that could max it out either, in fact there were "hidden" features like the detailed water and the perfect mode that got "unlocked" later on with patches and expansions, as PCs got faster.
I'm guessing something similar will happen with CoD as well, so that a lot of people will be able to run and enjoy it (thus ensuring good sales) but almost nobody will be able to have everything cranked up to maximum on release day, maybe not even with multiple DX11 generation cards in crossfire/SLI. After all, in most flight sims the real bottleneck tends to be the CPU in most cases...so even if i had dual 5970s on a core2quad i could be doing worse than a guy with a single 5850 on an 6-core i7.
But, like i said, it's just guessing
