There are a fair number of agnostics/atheists/other kinds of non-theists who oppose abortion.
Nat Hentoff, a columnist for the Village Voice, is probably the most prominent example. He is an atheist and also pro-life. You can see some of his commentaries on the issue here: http://www-swiss.ai.mit.edu/~rauch/nvp/hentoff.html
Doris Gordon, the founder of Libertarians for Life, is also an atheist. Her rationale for her views can be found here: http://l4l.org/library/abor-rts.html
Personally, my religious views have been in flux for my whole life (drifting somewhere between agnosticism and Catholicism), but I have never based my opposition to abortion on my religious views. You never hear much about non-religious pro-lifers, I suspect, because it doesn’t fit the agenda of either of the two biggest factions in the debate. Pro-choicers enjoy being able to stereotype pro-lifers as mere religious fanatics, and religious pro-lifers enjoy being able to turn many people’s queasiness about abortion into a chance to preach and convert people to their religious philosophy. Needless to say, I don’t agree with either of those agendas, so I’d really prefer if religion wasn’t an issue in the debate.
Regarding the discussion on this thread, my condolences on the tragic circumstances of your daughter’s death, Shagnasty. While I understand the justification for abortion in such a dire circumstance, I can imagine that being faced with the issue of aborting a 12 week old fetus would be devastating considering your views and your past tragedy. Have you discussed with your doctors if using IVF to pre-screen the embryos before implantation might be feasible? That seems like it would involve less emotional turmoil at least.