You can see a picture of the Kindle Paperwhite in landscape mode
here (link to a picture in the Customer Images Gallery on Amazon).
I haven't actually tried it, but one thing I noticed when I first got mine and played with the options a bit was that the Paperwhite seems to only allow either portrait (in one direction) or landscape (in one direction), unlike my Kindle 3 which allowed to change the orientation in all four directions (i.e. including upside down, with the keyboard on top, and in landscape having the keyboard on either side).
On the one hand I understand that it would be less useful with the Paperwhite form factor, as the sides (top/bottom and left/right) are much more similar in size than on the Keyboard, but it does mean that it's impossible for example to have the lights on top, which some people might prefer, considering their possible slight unevenness in lower surrounding light / when turned up high.