As you probably know, on the iPad2, the screen rotation lock can be set either by the button that's next to the volume control - the 'hard' switch - or by a 'soft' switch in the far left of the multi-task bar - which you can access by double-pressing the Home button and then flicking the screen right. The soft switch is at the far left.
Depending on how you have it set up in the 'Settings' app, one or other (but not both) of these control the screen rotation lock.
In addition, some apps can activate screen rotation lock too.
Tim