How about a triple click of the home button to change the orientation? Very simple, very practical. Apple already uses triple-click on the inline headphones. Just tried triple clicking my home button, and it's not a stretch at all, it would work very well. Such a feature would only work on a use-by-use basis so users don't get confused or permanently reconfigure the normal accelerometer setting, meaning when the accelerometer is moved, the orientation adjusts to the default for that position.