The physical resolution of the new MBP Retina is 2,880 by 1,800, but in fact at the highest resolution setting it actual uses a logical resolution of 1,920 by 1,200.
The mismatch is explained by this: For normal use, the screen uses more pixels to render text and graphics, keeping them at a readable size, but more sharp and crisp.
Games and other software can utilise the full resolution if needed. The article below explains more.
This isn’t quite what I was expecting, but then anyone who’s seen the difference between the new iPad 3 Retina screen and the old models will realise that Retina is the way to go.