Beside using the built-in software option to switch on and off the GSM/GPRS module of your Pocket PC Phone Edition, you can also use the hardware buttons.
Just press the red "telephone receiver" button of your device longer than 3 seconds and release it. Your PPC Phone Edition will shuts down the wireless module or start it up again and switches to the PIN entering screen.
This works with the o2 xda, T-Mobile MDA, VoiceStream Phone Edition and HP Jornada 928 WDA and is a fast and convenient way to stops working the wireless module, for example if you are ready to enter an aircraft. However, please make sure that the antenna symbol shows a disabled radio.
Cheers ~ Arne