Well this thread has raised a lot of comment to which I'd like to respond, but for the sake of staying on topic, I'll probably soon start an abortion debate in the general apologetics forum.
To answer to the original question, no, as pro-lifers and/or Christians, we should not use physical violence to protect the unborn.
The illustration of war is wrong in that if you're going to attack the source of the problem you need to go right back to the educational system, the government, the supreme-court, the doctors who (along with the media) cover up the truth of what they are doing to the unborn... in other words, society as a whole is responsible, be it actively, passively or permissively for the death of the unborn.
The abortionist is the instrument of the child's death. But if he is gone, another abortionist will take his place in killing the child, even if her mother must travel a little farther to have her killed.
Even if you could simultaneously kill all abortionists, don't think for a moment there won't be others willing to step out to do the work.
That is why, those of you who said education is the key, are right on. People are so sure the unborn are not human and that women have a right to abortion, that they some would risk their own lives to see the act protected at all cost. That's why it's so crucial, especially since media aren't on our side, that we work to get the truth about the unborn and about abortion out to women and men. Only once we've made abortion unthinkable will we ever stand a chance of seeing it end.
On that note, killing abortionists is actually counter-productive. Thanks to abortion-related violence and threats, many states and provinces have put 'bubble zones' into place that forbid protest of any kind within 50-500 feet from an abortion clinic (legal distance varries). This renders what was once an extremely effective means of giving a woman last-minute info that she might not yet have heard, before she makes her final decision inside the clinic, now nearly impossible (even from across the street, gently handing a woman a pamphlet or talking to her is not possible... shouting to her is not desirable at all, and rarely effective.
So because some chose to kill or threaten abortionists, countless unborn are being killed who once would have been saved, because no one is in front of the clinic to talk to their mothers about the choice they are about to make. (Not to mention that the whole movement looses credibility and gets painted as violent hypocritic fanatic murderers - who would listen to a maniac?)
So if these are not good reason to fight abortion on the intellectual front, where its roots are buried deep with misconceptions, I don't what is.
Kristine