This is an extremely important point for my strategy, which is to push the naturalist (about the resurrection) into relying on hallucination theory, and then showing the problems with that. I really do think the hallucination argument, allowing for a significant amount of legendary accretions in the story, is the best shot the skeptics have in explaining the founding of Christianity. While I can see memory distortion being the cause of thinking a cab was blue when it was really green, I have trouble with the idea of retroactive memory distortion accounting for someone thinking they saw someone whom they had seen executed two days earlier, if they didn't hallucinate and there was no resurrection.
Let me take a personal example. Bob Prokop, a frequent commentator here, was a friend of mine my days as an undergraduate at ASU in 1973-1974. We both had a close friend by the name of Joe Sheffer, who, tragically, passed away in 1989 at the age of 36. Now, I can imagine, in a crowd, seeing someone at a distance whom I thought looked just like Joe. But no amount of memory distortion could possible convince me that I had lunch with Joe in 2006, getting his take on the argument from reason, the state of contemporary Thomist philosophy, Thomist models of artificial intelligence, modern physics, the flaws of the Bush administration, and the latest debates on Dangerous Idea. No, if I really thought I had lunch with Joe in 2006, I would have to have been "appeared to Joe-ly," I would have to have had some Joe-experience, which, on the assumption that Joe didn't come back to life in 2006, would have to be a hallucination.
Note this assumes that putative eyewitnesses did actually martyr themselves, that this is a historical fact. I'm not a historian of Christianity, so am willing to play along with such assumptions.
What is required isn't strictly speaking martyrdom, but martyrdom risk behavior. Peter wasn't killed as a result of what he said outside the gate in Jerusalem, but what he said was inflammatory enough to the people who were responsible for the death of Jesus to expose him to the likelihood that he would be killed, and Peter knew it.