If you've updated to the latest and greatest version of iOS for the iPad, you're probably fuming at the disappearance of the hardware screen orientation lock. Luckily, the feature still exists, it's just hidden in the iOS software.
To lock the screen orientation in iOS 4.2, hold your iPad in the orientation you want to lock, and double-click the home button to bring up the task switcher. If you slide your finger to the right along the task switcher, you'll get a new screen with some playback, brightness, and volume controls, as well as a button on the left that will lock your screen orientation. It's certainly a bigger pain than it used to be, but at least the feature's still there. The hardware button now acts as a mute button instead.
For the record, this same trick works on the iPhone and iPod touch too — and, while we'd argue that the iPad is a bit different and people are more likely to want to lock their screen orientation quickly than mute it (since it's used for different things), if you're an all-around iOS user, this will make everything a bit more familiar across platforms.
How to Lock the Screen Orientation on Your iPad (with iOS 4.2) [The How-To Geek]