We don't usually report on book news here, but seeing as how I've already pre-ordered my Harry Potter book (and I'm sure many of you have), the first news story I clicked on today was regarding this!
So here you go,
The British/Canadian cover is the main one. The American cover is the top right and the adult cover is the bottom right.
(Taken from http://inhome.rediff.com/movies/2007/mar/28harry.htm)
To see them a bit better, Coming Soon has large pictures, specifically of the American cover.
The Canadian/British one looks more exciting (albeit, a little weird. Are they coming out of the Stargate?), and it's the book I'll be getting!