Just to clear things up, since I'm pretty sure that most of you on TOL have absolutely no idea what this means, let's first clear up what this does not mean:
It is not the case that abortion previously was an unforgivable sin.
In order to be forgiven of a mortal sin (i.e., any thought, word or deed which seriously violates the law of God), one must receive sacramental absolution. In other words, firmly intending no longer to commit the sin, one must go to a priest and confess one's (at least mortal) sins (the mortal sins must be confessed in kind and number), through the priest (who acts in the person of Jesus) to God. The priest, in turn (once more, acting in the person of Jesus), forgives the penitent.
Every sin is able to be forgiven.
The problem is quasi-jurisdictional. Certain sins carry a sentence of automatic excommunication from the church (i.e., the inability to receive the sacraments). In order to be forgiven of those sins, it is necessary to receive absolution from, e.g., a bishop. The parish priest doesn't have the jurisdictional authority to forgive those sins or to lift the resultant ecclesiastical sanctions. The bishop (or, otherwise, a higher ecclesiastic authority) does.
What the Pope has done is basically say: "You don't have to go to a bishop or have your parish priest receive permission from the bishop to forgive you for committing the sin of abortion. Your parish priest can do it."