The new iMacs have fewer vertical pixels because the aspect ratio is 16:9 instead of 16:10. Obviously, the iMac has a larger dot pitch than the MacBook Pro as well, as a result of its larger physical dimensions. The aspect ratio change is because home videos are typically 16:9, so things a better aligned for consumers than they are professionals (i.e. a 27" Display can show four 720p movies tiled, at 100% zoom, with no extra).
Vertical pixels are one of the earth's scarcest resources. I'd do almost anything to add another 160 vertical pixels to my cinema display (and soon, iMac), but alas it is not meant to be.