I've never been, but I hear that the west Coast (Oregon, Portland) is the place to be int he summer, I have a buddy that travels every summer to windsurf up there. I've heard La Ventana, Los Barriles, La Paz, that area is happening around that time, starts to pickup for the winter. Never been, but always heard a lot of my windsurfing buddies go there.
OMG how could I've forgotten, Lake Calima, in Cali, Colombia, is windy there like all year round, but I best winds are in the summer. I went a summer but I didn't used to kite then and the windsurfing was spectacular, haven't been back in 2 years, but maybe this summer. And is suuuuuper cheap.
http://calimadarien.com/eng_index.htm