Ever wanted to use the phone's green Talk key (sometimes called the Send button) in your application?  You can, but the docs are pretty well hidden, so I thought I'd point them out: http://msdn.microsoft.com/library/default.asp?url=/library/en-us/APISP/html/sp_porting_guide_qgcc.asp?frame=true

Basically, your app sends a SHCMBM_OVERRIDEKEY message, once, to its own menu bar telling it "When the user presses the Talk button (when my app is foreground) I want to recieve that as a WM_HOTKEY from now on."

Obviously, the most common use for this is to initiate a phone call to a specific number from within your app (using PhoneMakeCall()).   I wouldn't recommend using it for something non-phone-related.

On Smartphone, you can also override some of the other hardware buttons like Volume Up and Down.

- Jason Fuller