Just checked out my HTC Diamond to communicate to an ASP.NET app using ASP.NET Script Callbacks (that is without postbacks). The Diamond does do ASP.NET Script Callbacks under Opera web-browser and, most probably, will do it under future IE mobile.
It would be nice if ZigBee host (or any other low power low speed wireless host) were added to a mobile, and were found the way to make the wireless host be accessible for a web page's Javascript (mission almost impossible, I know).