The Cardix programming environment (see a nearby thread entitled "An easier way to write software that interacts with your vehicle" for more info) runs on the .NET Compact Framework, so this would enable you to write an application that runs on Windows Mobile and has ready access to OBD data. I know there are at least a few vehicle connection devices which provide Bluetooth connectivity. One which I have worked with a bit is the OBDKey device (
www.obdkey.com) and Cardix support for that device is in progress.