Juist a quick post so I won't forget this link: http://graegert.com/?p=505
Steve Graegert has written a great post on how you can work with the SPPropertyBag to store configuration settings. Of course that's very interesting, but in this post he also describes how you can build a custom Application Page that looks like it's an OOB Application Page. The trick is to know the SharePoint user controls that are responsible for building the UI, like InputFormSection and InputFormControl. Once again, check Steve's post for the full details!