Without wishing to play down the information Andrew has, in the absence of any press release from MAN B&W or Maersk, a lot of uncertainty remains over the exact nature of the propulsion systems.
The S-series engines are quite old and don't really fit in with Maersk's claim of a new, ultra-efficient ship design. One the other hand, the G-series is a brand new ultra-long stroke design which, on paper, seems far more suitable and especially compatible with the large, slow rotating props which have been specified. Some articles are claiming that the engines are only 6-cylinder, however, there is photo evidence to confirm that they are, in fact 8-cylinder
http://gcaptain.com/worlds-largest-ship-photos-maersk-moller/ (scroll down)
Some sources (and I am reliably informed) are suggesting that the engines are the 8G80ME-C9.2 type which, frankly, I would put my money on.
Brgds
Phil