I just found this doing a search and it works. The writer is anonymous:
It is called `portrait lock.` you can access this by double clicking your home screen button. This will bring up your active apps in a tray list. If you scroll all the way to the left you will see five icons (arrow circle , last track, play/pause, next track and iPod). If you accidentally choose the `portrait lock` you choose to make your phone display in portrait style only, no landscape orientation. If you have done this, just tap the circle with an arrow and you should be set! The way this is accidentally turned on is usually when you are choosing to use your iPod app an mis-tap the last song icon. I hope I helped
Read more: What does a lock icon with a circle around it mean iphone - I have a lock with a circle around on my iphone 3gs can you tell me what that means :: Ask Me Fast at
What does a lock icon with a circle around it mean iphone - I have a lock with a circle around on my iphone 3gs can you tell me what that means :: Ask Me Fast