'Full HD' (aka 1080p) is basically a resolution of 1920 x 1090 on a standard 16:9 ratio.
The 15" rMBP's screen is 2880x1800 pixels. So basically it's MORE than HD.
The resolution is then halved, to become 1440 x 900 as the default visible display - this is done so that if you set the screen to 'retina mode' (which its on as default) it shows a 1440x900 resolution desktop, so everything is a decent size, and easy to read. But in the background its actually running at 2880x1800.
If you were to watch an HD movie on it, the HD move is actually enlarged as its smaller than the screen.
To see what I mean, look at the video here (and skip to the 5 min mark)
http://www.apple.com/macbook-pro/#macbookpro