I use my iPad constantly on portrait mode (so much easier when it comes to reading on iBook) and only on landscape on certain games that require it and whenever I watch a movie/Netflix.
I'd not thought about it before, but rotate depending on what I'm doing. Since I thumb type mostly, it's in portrait right now. In the car I have a holder and it's easier to use in landscape. I use gps heavily. I don't know that I have a favorite position. It's in portrait when hanging at MR
Almost always landscape. Portrait use obviously when there is no choice (game or whatever). Only time i choose to use portrait is when I use my iPad as notes for speeches or if i read a book on it.
Still irks me that all the App icons change position when you rotate, there was a great jailbreak tweak ages ago that made it uniform when you rotated (I no longer bother with jb).
Almost always portrait. I like to pretend like it is an electronic piece of paper and that just feels more natural.
One of the reasons I switched to the iPad instead of android tablets was because I felt most large android tablets are optimized for landscape. I hate that.
90% of the time it is in portrait mode. The only time I ever really use landscape is when I am using YouTube but even then I still use portrait some of the time.