On a pretty new Galaxy S2+, the home screen used to rotate when the device was rotated. After switching the wallpaper of home screen (and perhaps lock screen as well), using the Photo app shipped with the device, the screen no longer rotates. Not even after restoring the original wallpaper, and not after rebooting the device.
To set the wallpaper, I use the corresponding entry in the Settings. I then get a choice of target, whether I want to affect home screen, lock screen or both. When I choose both, I get a choice of Gallery, Live Wallpapers and Backgrounds. When I choose home screen only, then I get the same three but Photos as well. So that's the reason I believe I might have set both. All the item names were translated by me, the actual English versions might be called slightly differently.
How can I restore the factory default of an automatically rotating home screen, without completely resetting the device to factory defaults?