I just upgraded my iPhone 3GS to iOS 5, and played around looking for improvements.
Exploring the assistive touch panel, I found the 'rotate screen' options work also when portrait orientation is locked. Effectively, we can select any of the 4 possible orientations, and it will stay locked while moving and rotating the device. Switching apps will cause a reset to default portrait orientation.
While a bit complicated to reach, this will still be useful. I'm hoping for a shortcut or dedicated way to activate, like quad-clicking the Home button to rotate right, for example. I like the triple-click to activate white-on-black, for a basic night mode.
All of these and more options are available under Settings » General » Accessibility and enabling 'AssistiveTouch.'
[crarko adds: There are indeed a lot of useful settings in there. Explore them. It looked slightly different on the iPad, and the 3GS may have different settings from iPhone 4.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20111015015549389