Phone usually portrait, unless I'm taking a photograph (which will be landscape as god intended, unless there's a very good reason not to), or watching a video, which I pretty much never do on a phone.
I have a 10" 4:3 fondleslab, and will use it portrait for reading books and landscape for watching video or when the Bluetooth keyboard's attached (or if there's power/headphones plugged in in a position that makes it awkward to rest the lower edge on something). Otherwise, it's square enough that either way round works pretty well for most purposes.