I just recently finished reading Infidel, the autobiography of Ayaan Hirsi Ali and I can't recommend it highly enough. The story of this amazing, courageous woman is a compelling read (for those not familiar with her, you can read more here or at her website).
In her book, Ali makes reference to the oft-heard claim that Islam is a "religion of peace" and takes some issue with that claim by pointing to information in the Quran and elsewhere that seem to belie it. Of course, it's often difficult to separate religious from cultural influences, especially when they appear to be so closely bound together as in most Islamic countries, but her arguments are interesting.
But what does it really mean to say "Islam is a religion of peace?" Do proponents of this claim mean to argue that Muslims are peaceful people, or that the religion itself is about peace or has peace as one of its primary doctrines? The former would seem to be trivially true, at least for the great majority of Muslims, but I don't think that really captures what people seem to mean when they argue that "Islam is a religion of peace". They seem to be saying that the religion itself is "peaceful" or that it is concerned in some way with peace. But it seems to me that this is no more possibly true of Islam than it is for any other religion. Because religions aren't at all concerned with peace...they're concerned with Truth.
Now I'm sure you can find passages in any given holy book(s) that talk about loving one's neighbor, but that says nothing about the ultimate concern of the faith, merely what the religion's founder(s) believe should be an outgrowth or a result of its practice. But this is often in direct opposition, or at least to some non-trivial extent contrary, to the central concern of any religion which is always Truth: what, at base, is existence really all about?
The striving for Truth seems to be deeply rooted in the human condition. We all want our worldviews to rest on a secure foundation and you can't get more secure than ultimate, absolute truth (what I characterize as Truth) and all religions claim to have it (this happens to be one reason why I believe religion are and have been so popular throughout human history).
But Truth is, by its nature, exclusive. Though all religions claim to have it, there is only one Truth and to the extent that religious truths conflict, only one (at most) can be correct. So religion is in turn by its nature, divisive. It divides us into "Right" and "Wrong". And the great majority of religions postulate terrible consequences for being "Wrong". Christian & Muslim doctrine condemns the "Wrong" to eternal torment. Buddhist & Hindu doctrines hold that the "Wrong" will suffer eternally via a cycle of rebirths. And so on and so on. According to these faiths, it's of the most extreme, eternal, and thus ultimate importance that one be Right, not peaceful. Of course, there are many passages in their holy books that can be interpreted as saying that peace flows from being Right, but it's still the case that the ultimate concern is not peace, but being Right.
The concern with being Right isn't exclusive to religion, of course. As I noted I believe it's human nature to be concerned with Truth. But this concern for being Right, be it religious or not, has directly led to some of the most horrible atrocities every perpetrated by Man upon himself. The Inquisition, the Reformation, the Terror (French revolution), pogroms and the Holocaust, the Cultural Revolution, etc, etc.
Its obviously not just religion that leads to such things, but religions, being ultimately concerned with Truth and separating the Rights from the Wrongs, are fertile breeding grounds for just this very sort of behavior. And thus it is that any claims made of the form (X is a religion of peace) should be taken with a grain of salt. Religion simply isn't about peace and its natural results are often opposed to it.