My friend Razib, writing as David Hume at the Secular Right blog, has looked at the data on international attitudes towards abortion and made some intriguing observations, notably:
1) It is clear that religion correlate with opposition to abortion in the vast majority of nations.
2) But, the attitudes of religious people and non-religious people track each other so that the irreligious in nation X may oppose abortion much more than the religious in nation Y.
That second point is really quite thought-provoking. It certainly underscores the moral subjectivity of abortion relative to issues like murder which have a clear and universal moral concensus.