Thanks Graham,
Well the thing is, if you treat the spindle as an Axis, you wouldnt have to worry about that.
Both movements would be automatically syncronized to finish at the bottom of the hole, just like any other XY move, except this time its a Z A movement.
Im not saying that the tapping would be lightning fast, but it would be possible.
Maybe just write a wizard to calculate the number of turns given a tap size and pitch and hole depth.
Regards
Fernando