Meh. Sorry, fellow atheists, but this is a tempest in a teapot. Imho, finding "contradictions" between the four Gospels is akin to finding "contradictions" between any four accounts of Leda and the Swan. Obviously, all the Gospels are variations on Mark, and are just the sort of variations one might expect in oral traditions. Most likely, there's one story that is the origin of all four Gospels- in that, I agree with the apologists.
The real problem is twofold: why are all four Gospels regarded as the Gospel truth, when they are not logically reconcilable; and why should we accept the truth of the original story, whatever it was? People make up stuff all the time, and embellish upon old stories, do they not? In the absence of evidence for the truth of any of the Gospels, while it's an interesting question for literary scholars what the original story said, it needn't concern us in our search for what really happened, unless there's independent evidence for its truth.
Whether Mark came first among the four canonical gospels is a disputed issue. And other factors would have to be taken into account, such as whether there was an earlier document written by Matthew and what form it took. But we can agree with Zilch that the gospels are variations on the same general theme. Even if we disagree on issues like whether the author of the fourth gospel was writing from his own memory of events or was retelling a tradition about events he didn't witness, we can agree that all of the gospels were derived from one core tradition in some sense.
We've addressed the issue of why we should trust that core tradition many times. See, for example, here. Since Rhology is posting at a blog that's addressed the subject so many times and in such depth, it isn't much of an objection to point out that we would need a reason to trust the core tradition. We've been arguing for the core tradition for a long time. So have other Christians. If skeptics want to advance the discussion, they should give the reader some idea of why they disagree with Christian argumentation on the subject.
All of us harmonize. Historians do it in other contexts, not just in the context of early Christianity. Evolutionists harmonize theories about how evolution occurred in one context with theories about how it occurred elsewhere. Atheists who argue against Christianity harmonize their theory about one aspect of early church history with their theory about another aspect of it. Or if they're addressing modern paranormal claims, for example, they adjust their explanation of one piece of evidence to fit with their explanations of other data.
The degree to which we harmonize is going to vary depending on the nature of the case. If you have reason to believe that two documents are a Divine revelation, then it's going to make sense to harmonize those two documents to a higher degree than you would harmonize two documents for which you have no such evidence. If other relevant factors are involved, such as an acknowledgment by the authors that they're giving contradictory accounts, then you'd have reason to harmonize to an even lesser degree.
In the case of the gospels, there are reasons to harmonize the documents to some extent before we even get to the point of harmonizing them as much as we would if they're Divine revelation. We have some early sources, like Papias and some elders referred to by Clement of Alexandria, who give us information about how the gospel authors viewed each other. We would also look at how the early Christian and non-Christian sources viewed the relationship among the gospels and their authors. For example, we have early heretical and otherwise non-Christian sources (Apocryphon Of James, Marcion, Celsus, etc.) who acknowledge facts such as the traditional authorship attributions of the gospels and a close relationship among the authors of those documents. Modern skeptics often argue that Matthew and Luke used Mark as a source, which suggests that the authors held a high view of Mark. If they held a high view of Mark, then charges of contradiction among them should be adjusted accordingly. We would also have to ask how likely it is that the authors who wrote last would have been unaware of the earlier gospels or would have held a low view of one or more of the earlier gospels without that low view having left more of a trace in the historical record. We also have to consider the reasons authors would have had for making their accounts different even where they agreed. Luke varies his accounts of Paul's conversion when he addresses the subject three times in Acts, for example. Ancient authors often did that. They varied their accounts by including details in one rendering that they didn't include in another rendering, looking at the event from a different angle one time than they did a previous time, etc. That sort of variation was considered good literary practice. It could involve contradiction, but that sort of variation is going to produce more perceptions of inconsistency even if there aren't any contradictions. And authors would sometimes supplement one another, sometimes assuming a previous author's claims without stating them, all the while adding material that the previous author didn't mention. Authors are often silent about something because they're assuming it, not because they're ignorant of it. Issues like these have to be taken into account before we even get to the issue of whether the documents are Divinely inspired and, therefore, are deserving of harmonization to an even larger extent.
If a skeptic doesn't think the documents are Divine revelation, then he's not going to think they deserve as much harmonization as a traditional Christian thinks they deserve. But the skeptic, if he's going to act responsibly, still has to try to discern to what extent harmonization is warranted apart from a belief in concepts like Divine inspiration and inerrancy. A traditional Christian view of the Divine inspiration of the gospels isn't the only reason to harmonize them. A good recent source that covers many of the issues related to this subject is C.E. Hill's Who Chose The Gospels? (New York: Oxford University Press, 2010). See my comments on the book here.
If we narrow our focus to the resurrection narratives, we would have to take factors into account that are relevant to those passages in particular. For example, what's the genre of the resurrection narratives? On what issues do two or more of the accounts agree? Are the points they agree on ones that are unlikely to have been fabricated? Is it likely that the earliest Christians would have been interested in preserving resurrection accounts from the witnesses? One issues like these, see here, here, and here.