You can lock the screen rotation using the slide button near the volume (you'll have to make sure that this button is configured for lock rotation in your settings).
Or just double click the home button, the taskbar will show. Slide the task bar to the right, showing ipod controls. At extreme left, you have an icon locking the rotation of the display... That will be so if your side button is set to mute the sound instead of lock rotation...
See screen capture (sorry, it's in french)
VicoPad addict!