Mine's almost exclusively used in landscape. Especially for using Safari as I find the text difficult to read in portrait, but also most games and apps tend to work better for me in landscape.
Landscape usually. But what I don't like are apps that force to one way or the other. The device is supposed to be use it your way, but many developers force to one orientation. The UI guidelines indicate developers should allow, so don't know why Apple would allow an app to be used only one way?
It really is nuts how much Johnathan Ives is right. For me, I don't really think about the change from landscape to portrait. I just...change when I need to.
When I'm reading through Twitterific, I'm usually in portrait, same for browing. When I'm in Mail I'm usually in landscape. I don't even think about it....I just *insert CRAZY Scott Forstall eyes here* DO!
But, when I'm in portrait, I'm usually upside down. Home button on top makes perfect sense to me for some reason.