I don't think much about this question not only because I'm not a developer, but also because I never heard anyone say they avail themselves of such a thing. However...
If Windows Mobile is an open platform, such an API has to be available. Steve Ballmer, of all people, made me rethink this during his October 23 speech at CTIA Wireless IT & Entertainment in San Francisco. Quoting Steve (my emphasis in bold):
"We're investing very heavily in the device itself, and in the services around them as a platform. If you want to write a rich application, we have a version of .NET that runs on Windows Mobile phones that supports rich application development. If you want to write a thin client application, HTML, or AJAX, or eventually with our Silverlight technology which provides for rich media and video, those things will be available on this platform. We have rich APIs for things like forms and Web services, for location, for contact, calendar, messaging, maps, sound, graphics, all of that is available for third party innovation."I attempted to learn more after Steve's speech from Microsoft's PR agency, but they didn't respond to my inquiry. Anyone reading this know what he's talking about?
Update: See my later post for the answer.