Sin is as alien to the contemporary mind as fetching water from a well, darning your own socks or finding Demis Roussos sexy. According to the Catholic Catechism, sin is "humanity's rejection of God and opposition to him", which of course means that the godless (a bracket into which a large number of generation Y will fall) find the whole notion irrelevant, senseless or both. This is precisely what Christians who accept the idea of sin find deeply disturbing: a culture that doesn't even care about sin has truly cut itself off from God's grace and is therefore sinful in the most profound sense.
Well, I don't find it disturbing, since it's exactly what one would expect. But what I want to focus on is not that but the confusion. Baggini confusedly takes a description for a definition in his appeal to the Catholic catechism. From a Christian perspective, every sin will be a rejection of and opposition to God, but it doesn't follow from this that that's what the term 'sin' means. Claiming that every sin is properly described as a rejection of and opposition to God is not the same as claiming that that's what the term 'sin' means. One might as well say that, because a credit card can be described from a certain perspective as a convenient form of payment, the term 'credit card' means 'a convenient form of payment'. To distinguish descriptions from definitions takes serious analysis. It is also one of the oldest and most venerable parts of philosophy as we know it, since the distinction of mere descriptions from definitions was the whole point of Socratic investigation. I refer you to Plato's dialogues if you need a refresher on the subject.
Sin is a morally bad act; it is to action what vice is disposition. Had Baggini taken a more technical definition, such as Aquinas's (deriving from Augustine), which in one form or another is very popular, he wouldn't be so quick to assume that 'sin' always means 'contrary to God's will' when Christians talk about it. For 'sin' in that sense is a word, deed, or desire contrary to eternal law -- and so human sin is word, deed, or desire contrary to our little bit of eternal law, namely, the authoritative moral dictates of practical reason. And, last I heard, most atheists hadn't foresworn practical reason or the authority thereof. So it's unfortunate that Baggini has chosen to dabble in equivocal word play rather than serious analysis or substantive argument; people have a right to expect more from philosophers in public view, and, as it is a purely regressive contribution to the discussion, it doesn't do anyone any good.
UPDATE: Here is the section from the Catechism to which Baggini refers (386):
Sin is present in human history; any attempt to ignore it or to give this dark reality other names would be futile. To try to understand what sin is, one must first recognize the profound relation of man to God, for only in this relationship is the evil of sin unmasked in its true identity as humanity's rejection of God and opposition to him, even as it continues to weigh heavy on human life and history.
Notice that there is no claim here of defining sin; rather, the claim is that no one fully understands the evil of sin until they recognize that all sin is humanity's rejection of God and opposition to him. This is confirmed by the very next section:
Only the light of divine Revelation clarifies the reality of sin and particularly of the sin committed at mankind's origins. Without the knowledge Revelation gives of God we cannot recognize sin clearly and are tempted to explain it as merely a developmental flaw, a psychological weakness, a mistake, or the necessary consequence of an inadequate social structure, etc. Only in the knowledge of God's plan for man can we grasp that sin is an abuse of the freedom that God gives to created persons so that they are capable of loving him and loving one another
Notice that the point is clarification and clear recognition of sin, of avoiding mistakes in explaining sin; there is no attempt here to define sin. In other words, the intended idea is that we are here dealing with a true description; it isn't put forward as a definition. When the Catechism actually sets out an informal definition of sin (1849), it adapts and quotes Aquinas's.