Windows – How to “Connect via Bluetooth” in WM programmatically

I would start looking in MSDN Windows Mobile 5.0 documentation for the Blue Tooth APIS.

The changing of the Audio Device to the Bluetooth device is likely to be the hard bit. Depending on what application you are trying to switch between you may like to look into Waveform Audio for Windows Mobile-based Devices API.

Having never done any of the above, I can't say if it's enough but it should get you started.

Even if you join the MFi/WWi developer program, it's impossible to connect your iPhone to an arbitrary printer. The printer would need to built specifically for iPhone compatibility and incorporate an Apple-specific chip. The program is intended for device manufacturers to build iPod/iPhone specific devices, not for independent developers or device manufacturers with existing bluetooth products.