This FAQ applies to player models that don't support Portrait mode (HDx20, HDx10, LSx22). See this FAQ for more details.
Let's assume want to display a 1920x1080 video in portrait mode. You can create your content “sideways” from your authoring application (e.g. Photoshop, After Effects, Quicktime Pro, etc). When you are done editing, you must output your content sideways as 1920 x 1080. It’s then presented right-side up when the display is in portrait mode.