The Mobile Secretary is an application that will let your Windows Mobile device automatically respond to a missed call with a custom text message and optionally with your current calendar information. Mobile Secretary comes to us courtesy of the Windows Mobile Team and is available for free. You can also set up different groups and assign different messages to each. It also does a nice job covering situations where someone might call you from a home phone or situations where you have more than one mobile phone.
The Mobile Secretary requires Windows Mobile 5.0 and the .Net Compact Framework v2.
The Windows Mobile Team will also post the source code for the application fairly soon.