I've stumbbled upon a request of an application that will automatically turn on the loudspeaker when the phone is away from the ear, and turns it off when it is near the ear. I really liked the idea and this is the result.

AutoLoudspeaker is fairly simple to use. When running the application a service starts. If the shortcut is pressed again, a window shows and the user can choose to close the service. As long as the service runs the loudspeaker will automatically switch on if the proximity sensor is unblocked and off if it is blocked (only during an active phone call).

I wrote the application and tested it on my HTC HD2, so I can't say how it will operate on other devices...

Main features:
Switch the loudspeaker according to the proximity-sensor or g-sensor.
Answer an incoming call by gesture: wave your hand in front of the proximity-sensor.

Further information may be found on the applications thread on XDA:
AutoLoudspeaker Homepage