The reason I've been holding off on including this is the need for deciding whether it's an optimization to the existing digitalWrite() or an additional function. This seemed like something that made sense to target for the 1.0 release, but probably could have been (could be) resolved sooner.
In particular, I think we should simply remove the checking for and disabling of PWM output from the digitalWrite() function and use this optimized version. Then we will have, if I understand it, a one-instruction digitalWrite() for cases where the pin number and value are a compile-time constant. Anything slower than that seems like it might suggest an alternative (faster) option, which I think would be an unnecessary complication.