JLNobody wrote:I vaguely think of modernism and post-modernism as different facets of the revolt against traditionalism, a cultural dimension rooted deeply in the era of Christianity.
Yes, I think the closest one can get to a definition of modernism is that it is the belief that "being modern"--that is, being in the present--is something that one should aspire to. Not that this is a terribly profound statement. One of the problems with the concept of modernism is that it is called on to encompass such a wide array of trends that any definition that tries to be more specific than that is bound to exclude something.
Not that this has bothered many historians. Ask a literary, art or music historian what the "modernist" canon is, and then ask them what the academic canon is (or was, for most of the 20th century), and they are sure to line up. Figures who don't fit into the former have often been excluded from the latter.
That said, if the academic definition is taken to be the default definition of modernism, then I'm one of those who believe that postmodernism is more a continuation of, rather than a reaction to, modernism. As you mentioned, modernists and postmodernists often respond to the same sorts of crises of modernity--pluralism, the obsession with technical innovation and the loss of tradition. Postmodernists just do it with a little more humor.