I can't recommend Razib's enormously intelligent post on religion, secularization, and various associated topics highly enough, and Reihan's (somewhat more personal) response is likewise thought-provoking. I'd only quibble with Razib's remark that "Europeans are post-Christian, but not predominantly 'secular,' if that means lack of belief in God and a 'spirit or life force'" - not because he's wrong that a widespread "supernaturalism" prevails even (or especially) in the absence of organized religion, but because I think that it's worth employing a definition of secularism that doesn't conflate it with atheism.
In the forthcoming, not-yet-online Atlantic, for instance, I have a short piece analyzing the rise of mass secularism in America, which draws on this paper by Michael Hout and Claude Fischer on the remarkable growth in the percentage of Americans with "no religious preference." Hout and Fischer attribute this growth, in part, to people self-defining against organized religion because of its association with conservative politics, but (like Razib) they shy away from calling this phenomenon "secularism" because many of the "no religious preference" types retain supernatural beliefs. But I tend to think that the term "secularism" is actually most useful, and exact, when applied to a political hostility to organized religion of precisely the kind that Hout and Fischer are documenting, rather than to a more general disbelief in the supernatural. "Secularist" should be synonymous with "anti-clericalist," in other words, rather than with "unbeliever." (It seems like a poor definition of secularist that excludes Deists like Thomas Paine and Voltaire - or that excludes Sam Harris, for that matter, because of his forays into Eastern mysticism.)
There are two strains of secularism, I would argue, which are usually intertwined but philosophically distinct: A soft secularism that argues for a legal separation of church and politics - no school prayer, no federal funds for churches, etc - and a hard secularism that militates for a complete separation of religion and politics, and shades easily into hostility toward organized religion in a general. But neither form precludes private belief in the supernatural. A perfectly "secular" society would be defined not by universal atheism, but by a religion-free politics in the short run, and probably a long-run "decoupling," as Razib puts it, of supernatural beliefs from religious institutions.
The dictionary, of course, tries to have it both ways, like the squish it is.
Photo by Flickr user Dhammza used under a Creative Commons license.