You can scroll the entire screen up by exactly the amount needed to prevent the keyboard from obscuring the field. It doesn't even need to be on a scrollview.
You'd think the framework would do that (or at least offer that), but nope.
This is also a good approach, but if you do that, you have to remember two things. First, you can only reliably measure the height of the keyboard at runtime, and the keyboard size could change even after it was displayed (if the user touches the globe button to change the language of the keyboard). Second, if your app supports the iPad as well, you have to remember that on the iPad the keyboard is detachable since iOS 5 so make sure you won't have anything ugly (like a big empty black rectangle) where the keyboard would normally be.
You'd think the framework would do that (or at least offer that), but nope.