1. A method for implementing a custom user interface (UI) form in a personal information manager (PIM), the method comprising: loading a form markup file containing markup that specifies a layout of one or more UI controls on the custom UI form, data binding information specifying internal and external data sources from which contents of the one or more UI controls will be populated, and event subscription information specifying logic to be executed by a custom event handler when a particular event is generated by the one or more UI controls; generating a native UI form specification for the PIM 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; rendering the one or more UI controls on the custom UI form by a forms runtime engine external to the PIM; and displaying the custom UI form by incorporating the custom UI form in a specific region of a native UI displayed by the PIM.