These NYTimes instructions are for 'stopping' the rotation, but the reverse will 'start' the rotation again.
"You can lock the iPhone 4 screen orientation on the to the vertical “portrait mode” to keep it from changing. To do so, double-click the iPhone’s Home button to open up the bottom panel of apps. In the app panel, swipe your finger from left to right until you get to the first icon, which has a circular arrow in the middle. Tap this circular arrow icon — known as the “portrait orientation lock” — to lock the screen into the vertical orientation so it won’t move, even if you do.
To free up your screen and let it rotate freely again, reverse the process. (Shortcut controls for the iPhone’s iPod functions are also down in the app panel in case you want to skip, repeat or pause currently playing music without having to burrow into the iPod app on the Home screen.) When the portrait orientation lock is on, a small version of the circular-arrow icon appears at the top of the iPhone screen. Even when you have the phone locked in the portrait mode, some apps like YouTube videos or the Street Views feature of the Google Maps app still appear in the horizontal landscape mode.
Thanks and source: Q&A: Make the iPhone Screen Stop Spinning - NYTimes.com
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