This bit of information caught my eye today as I was reading about Feature Services in the ArcGIS Server .NET help:
Feature services also support HTML pop-ups setup in ArcMap. HTML pop-ups are a powerful and easy way to share HTML formatted information about features. They work much like the Identify tool, except that the information display can be customized HTML.
I set up the HTML popup in ArcMap and it worked as expected, and then I published a map service. The REST endpoint of my service now shows esriServerHTMLPopupTypeAsURL as the HTML Popup Type, however, I can't seem to find a sample, etc. that shows how to display the popup in a web application.
Edit: Thanks to those who supplied answers for the links. I think I need to re-phrase the question: I'm not having problems getting the popup to display - that is working fine. My HTML popup type is set as PopupTypeAsURL, so I was expecting the popup to display the contents of the URL, say http://whateversite.com. Essentially I want the popup to display a web page or web form. Is this possible?