With the release of iOS 4.2 for the iPad, the orientation lock hardware button is changed to a mute button. There is no way of changing the behavior of the hardware button back without jailbreaking your iPad.
Screen orientation lock is still available via the multitask bar, similar to iOS 4 on iPhone and iPod touch. To lock the screen orientation, rotate the iPad to the orientation you want to lock. Double-tap the iPad’s home button to bring up the multitask bar. Swipe the bar all the way to the right to bring up the orientation lock button and iPod controls, as shown in picture above. Simply tap the button to lock the orientation.