Mac Screen Rotate (http://www.macscreenrotate.com) is a new app for Mac OS X that allows you to rotate your display for portrait viewing with global keyboard shortcuts. It costs US$4.99; a demo is available for download.
Mac Screen Rotate also rotates the tracking of the built-in touchpad to align with the portrait screen orientation and switches the arrow keys. It allows you to, for example, hold the MacBook Air like a book and use it like an ebook reader.