Paul's summary is right on and I apologize for putting off so many of the decisions.
If we decide to remove the PWM checks from digitalWrite(), I would suggest adding a noAnalogWrite(pin) function which stop the PWM generation on the pin.
The thought was that if this change were to happen, it would be best to do in Arduino 1.0. In which case, it seems annoying to change the performance of digitalWrite() twice in quick succession: once for 0019 (optimizing it as Paul has implemented but keeping the PWM check) and once for 1.0 (removing the PWM check). But maybe the performance benefits outweigh the pain of having to tweak your code twice?