One of Apple's big shortcomings in the iPhone SDK is input methods. Apple gives developers very little support in terms of custom input methods, which is why it's so frustrating that the 10-button numeric keyboard that is provided in the SDK is missing something as crucial as a "DONE" button.
Thanks to the work of a development team called Neoos, many app developers found a workaround to this issue by adding a custom button on top of the default numeric keyboard. But in iOS 4, this method stops working. I did some digging and found out everything I could about the issue and want to share it with any developers who might be looking for the same solution I am.