Do you have the rotation lock enabled?
Sent from The Droid... The one you were looking for...
Yes - look at the rotation lock - it can either be the hard switch by the volume control or the soft switch in the multi-task bar (double press the Home button, then flick the screen to the right, the switch might be the one on the far left hand side. Whether it's the hard or soft switch is decided by your configuration in 'Settings'.)
This is almost certainly the problem that you have but, if not, get back in touch and we'll suggest some more things...
Tim
i had the same problem before even though the rotation lock was disabled. restarting my iPad did the trick.