1. A method for implementing a custom user interface (UI) form in a personal information manager (PIM), the method comprising: accepting a form markup file containing markup that specifies a layout of one or more UI controls on the custom UI form; generating a native UI form specification from the form markup file; storing the native UI form specification in a PIM form storage; receiving a request at the PIM to display the custom UI form; upon receiving the request, retrieving the native UI form specification from the PIM form storage; and rendering the custom UI form from the native UI form specification.