Look at the first part of John’s post. We see the phrase “if it is burdensome”. The situation described by John is a case where the priest has not been granted faculties to lift the excommunication. If the priest believes it is too burdensome for the penitent to remain excommunicated, then he can give a temporary reprieve. If, on the other hand, a priest has been granted faculties to lift the excommunication for abortion, then there is no further need to go to the bishop.
Thistle, this is a situation where mortal sin and excommunication do not necessarily coincide. As far as i know, and i could be wrong on this particular point, any priest who has been granted faculties to hear confessions automatically has the authority to grant absolution for any sin, including abortion. What the priest may not have is the faculties to lift the excommunication that is associated with it. So, in the situation described by John, the penitent is forgiven, and the excommunication is temporarily lifted (for a month), and the penitent has to go to the bishop, not for absolution, but to be re-communicated.
This situation is possible because mortal sin is a spiritual condition, while excommunication is a legal sanction.