After upgrading the iPad to 4.2.1 the screen rotation lock button no longer has any effect on the screen rotation. Instead it turns off the speaker. I've looked in settings but I don't seem to find any way to change this behavior.
At iPad: Understanding screen rotation lock, Apple says:
Beginning with iOS 4.2, the switch previously used for screen rotation lock functions as a silence switch for notifications and system sounds.
Sucks, doesn't it?
It is still possible to lock the screen rotation on an iPad (and that article explains how); there's just no hardware solution any more.