I use a variety of cases for each scenario, but a lot of iPhone owners keep their iPhone's naked. If you're one of those people, you might be interested in the iKlik, a plastic molded stand specifically built for holding a naked iPhone in landscape mode.
The iKlik could be useful for both viewing movies on a plane or also for using your iPhone as a GPS on your dash. They include adhesive rubber grips that will help keep your iKlik in place. You can also text, type notes and compose emails in landscape mode, at the most comfortable angle you choose, as the iKlik ratchets to five different angles.
The iKlik is a nice idea, and well made, as long as you understand that you can't use it with a case and it's for landscape mode only.