A month ago, I discovered how to lock the screen orientation on my Samsung Galaxy Tab. I then locked my screen on potrait mode... and forgot how to do this... :(
After various searches on the web, I finally rediscovered how to undo this. So I'll post it here so I won't forget.
I think this is somewhat device dependent.
In a Galaxy Tab, from just about any screen, you "pull down" the notification screen, and up there on the top right you'll see, "orientation lock" (either in color (On), or gray-ed out (Off)). Tap it. and you're locked.
See this URL for a Youtube demo http://bit.ly/oAC16U
But in another Youtube http://bit.ly/oExwOi, it shows that on an O2, its different, maybe an older Android OS. It is in SETTING -> Display -> Orientation. I'd say this is a more sensible place.
(This is part of my answer to this androidforum article.