13. A system comprising: a computer comprising at least one processor that executes computer executable instructions stored in a memory in communication with the at least one processor that, when executed by the computer, cause the computer to: load a form markup file that specifies a design of a custom UI form containing markup that specifies a layout of one or more user interface (UI) controls, 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; generate a native UI form specification for a personal information manager (PIM) from the markup of the form markup file; store the native UI form specification in a PIM form storage; receive a request to display the custom UI form; upon receiving the request, retrieve the native UI form specification from the PIM form storage; for each of the one or more UI controls having data binding information specified in the form markup file, bind a data source specified in the data binding information to the UI control; for each of the one or more UI controls having an event subscription specified in the form markup file, subscribe to an event specified in the event subscription to a custom event handler for the UI control; and render the custom UI form from the native UI form specification and display the custom UI form by incorporating the custom UI form in a specific region of a native UI displayed by the PIM.
13. A system comprising: a computer comprising at least one processor that executes computer executable instructions stored in a memory in communication with the at least one processor that, when executed by the computer, cause the computer to: load a form markup file that specifies a design of a custom UI form containing markup that specifies a layout of one or more user interface (UI) controls, 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; generate a native UI form specification for a personal information manager (PIM) from the markup of the form markup file; store the native UI form specification in a PIM form storage; receive a request to display the custom UI form; upon receiving the request, retrieve the native UI form specification from the PIM form storage; for each of the one or more UI controls having data binding information specified in the form markup file, bind a data source specified in the data binding information to the UI control; for each of the one or more UI controls having an event subscription specified in the form markup file, subscribe to an event specified in the event subscription to a custom event handler for the UI control; and render the custom UI form from the native UI form specification and display the custom UI form by incorporating the custom UI form in a specific region of a native UI displayed by the PIM. 16. The system of claim 13 , wherein the custom event handler is contained in a custom event handler module specified in the markup.